Tests your avatar in-editor from the VRChat SDK->Test Selected Avatars menu item.

VRChatHelper.cs

// VRChatHelper.cs, a tool to help test your model without launching VRChat.
// Made by naelstrof (naelstrof@gmail.com)
// Some eye tracking advice to help prevent this tool from yelling at you: https://docs.google.com/document/d/1BvX_OdEilbJ7wEcvd5MRA1g29NGCAp3G3nHGp73t_CQ/edit?usp=sharing

#if UNITY_EDITOR
using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEditor;

[InitializeOnLoad]
public class VRChatHelper : MonoBehaviour {
    static int testing;
    [MenuItem("VRChat SDK/Test Selected Avatars")]
    private static void TestAvatar() {
        foreach (GameObject obj in Selection.gameObjects) {
            VRChatHelper v = obj.AddComponent<VRChatHelper> ();
        }
        EditorApplication.isPlaying = true;
    }
        
    static VRChatHelper() {
        EditorApplication.playmodeStateChanged += PlaymodeStateChanged;
    }
    // This is called for each object, including the global static one. So we just keep track of how many have initiated/stopped to determine if we've stopped testing.
    private static void PlaymodeStateChanged() {
        if (EditorApplication.isPlayingOrWillChangePlaymode) {
            testing++;
        }
        if (EditorApplication.isPaused == false && EditorApplication.isPlayingOrWillChangePlaymode == false && testing > 0) {
            testing--;
        }
        if (testing == 0) {
            foreach (VRChatHelper v in Component.FindObjectsOfType<VRChatHelper>()) {
                DestroyImmediate (v);
            }
        }
    }
    Animator a;
    Vector3 lookPos;
    VRCSDK2.VRC_AvatarDescriptor d;
    SkinnedMeshRenderer m;
    SkinnedMeshRenderer eyeMesh;
    Transform le, re;
    int blinkl;
    int blinkr;
    int lowerlidl;
    int lowerlidr;
    int curindex;
    float waitTime = 1f;
    float waitTimer;
    HumanDescription h;
    HumanLimit leLimit;
    HumanLimit reLimit;
    Vector3 leStartRot;
    Vector3 reStartRot;
    Vector3 jawStartRot;
    bool sane;
    bool warn;
    bool eyesSane;
    public bool TestWalking = true;
    public bool TestEyes = true;
    public bool TestLipSync = true;
    void Start () {
        eyesSane = true;
        // Check that we're applied to the right thing.
        a = GetComponentInChildren<Animator>();
        d = GetComponent<VRCSDK2.VRC_AvatarDescriptor> ();
        if (d == null) {
            Destroy (this);
            throw new UnityException ("A non-avatar was selected! Ignoring...");
        }
        if (a == null) {
            Destroy (this);
            throw new UnityException ("It seems your avatar is missing an animator, you should add one! (It's a component, and required).");
        } else {
            if (a.runtimeAnimatorController == null) {
                // Try to quickly apply the simple animator...
                string[] guids = AssetDatabase.FindAssets ("SimpleAvatarController", null);
                if (guids.Length <= 0) {
                    throw new UnityException ("Your avatar needs an animation controller attached to the animator component! Try setting it to SimpleAvatarController.");
                }
                RuntimeAnimatorController rAC = AssetDatabase.LoadAssetAtPath<RuntimeAnimatorController> (AssetDatabase.GUIDToAssetPath (guids [0]));
                if (rAC == null) {
                    throw new UnityException ("Your avatar needs an animation controller attached to the animator component! Try setting it to SimpleAvatarController.");
                }
                a.runtimeAnimatorController = rAC;
            }
            string fbxAsset = gameObject.name;
            // Check bone limits.
            if (!GetHumanDescription (fbxAsset, ref h)) {
                throw new UnityException ("Failed to import the human description of the specified asset. Is it a humanoid, or did you change the name of the prefab in scene? (The avatar's name in the hierarchy must share its name with the model name in the asset browser.)");
            }
            foreach (HumanBone bone in h.human) {
                if (bone.limit.max == Vector3.zero && bone.limit.min == Vector3.zero && bone.limit.axisLength != 0) {
                    Debug.LogError ("A muscle attached to " + bone.humanName + " has 0 flexibility. This might not be intended. (To fix, adjust your muscle settings.)");

                }
                if (bone.humanName == "LeftEye") {
                    leLimit = bone.limit;
                    if (bone.boneName == "LeftEye") {
                        Debug.LogError ("You shouldn't have LeftEye be the real eye, it should be an empty bone! Create a different bone for eye tracking. (See https://docs.google.com/document/d/1BvX_OdEilbJ7wEcvd5MRA1g29NGCAp3G3nHGp73t_CQ/edit?usp=sharing for details.)");
                        warn = true;
                        eyesSane = false;
                    }
                }
                if (bone.humanName == "RightEye") {
                    reLimit = bone.limit;
                    if (bone.boneName == "RightEye") {
                        Debug.LogError ("You shouldn't have RightEye be the real eye, it should be an empty bone! Create a different bone for eye tracking. (See https://docs.google.com/document/d/1BvX_OdEilbJ7wEcvd5MRA1g29NGCAp3G3nHGp73t_CQ/edit?usp=sharing for details.)");
                        warn = true;
                        eyesSane = false;
                    }
                }
            }
        }

        // Check that our visemes are fully definined, if we're an avatar with viseme blend shapes.
        if (d.lipSync == VRCSDK2.VRC_AvatarDescriptor.LipSyncStyle.VisemeBlendShape) {
            m = d.VisemeSkinnedMesh;
            foreach (string s in d.VisemeBlendShapes) {
                if (m.sharedMesh.GetBlendShapeIndex (s) == -1) {
                    throw new UnityException ("Missing viseme definition, make sure they're all set in the Avatar Descriptor.");
                }
            }
        } else if (d.lipSync == VRCSDK2.VRC_AvatarDescriptor.LipSyncStyle.JawFlapBlendShape) {
            m = d.VisemeSkinnedMesh;
            if (d.MouthOpenBlendShapeName == "" || d.MouthOpenBlendShapeName == null) {
                throw new UnityException ("Missing jawflap viseme definition, make sure it's set in the Avatar Descriptor.");
            }
        } else if (d.lipSync == VRCSDK2.VRC_AvatarDescriptor.LipSyncStyle.JawFlapBone) {
            if (d.lipSyncJawBone == null) {
                throw new UnityException ("Missing jawflap bone make sure it's set in the Avatar Descriptor.");
            }
            jawStartRot = d.lipSyncJawBone.localEulerAngles;
        }

        // Check that the mesh "Body" exists. Required for eyes to track.
        Transform body = gameObject.transform.Find ("Body");
        if (body == null) {
            Debug.LogError("Mesh containing blinking visemes must be named \"Body\", or eye tracking won't work!");
            warn = true;
            eyesSane = false;
        } else {
            // Sanity check that our Body is actually a mesh
            eyeMesh = body.gameObject.GetComponent<SkinnedMeshRenderer> ();
            if (eyeMesh == null) {
                Debug.LogError("Mesh containing blinking visemes named \"Body\" isn't actually a mesh!");
                eyesSane = false;
                warn = true;
            }
            // Check that our blink visemes exist and are in the correct index.
            blinkl = eyeMesh.sharedMesh.GetBlendShapeIndex ("vrc.blink_left");
            blinkr = eyeMesh.sharedMesh.GetBlendShapeIndex ("vrc.blink_right");
            lowerlidl = eyeMesh.sharedMesh.GetBlendShapeIndex ("vrc.lowerlid_left");
            lowerlidr = eyeMesh.sharedMesh.GetBlendShapeIndex ("vrc.lowerlid_right");
            if (blinkl != 0 || blinkr != 1 || lowerlidl != 2 || lowerlidr != 3) {
                Debug.LogError("Missing blink shapes, need to be named correctly and in the correct index for eye tracking to work!");
                eyesSane = false;
                warn = true;
            }
        }

        // Make sure our armature has the correct structure for eyes to work.
        Transform leftEyeCheck = gameObject.transform.Find ("Armature/Hips/Spine/Chest/Neck/Head/LeftEye");
        Transform rightEyeCheck = gameObject.transform.Find ("Armature/Hips/Spine/Chest/Neck/Head/RightEye");
        if (leftEyeCheck == null || rightEyeCheck == null) {
            Debug.LogError("Armature structure must be exactly `Armature/Hips/Spine/Chest/Neck/Head/{LeftEye,RightEye}` in order for eye tracking to function!");
            eyesSane = false;
            warn = true;
        }
        le = a.GetBoneTransform (HumanBodyBones.LeftEye);
        re = a.GetBoneTransform (HumanBodyBones.RightEye);
        if (le == null || re == null) {
            Debug.LogError ("Eyes were not set in the rig configuration! Eyes won't work unless they're specified.");
            eyesSane = false;
        } else {
            leStartRot = le.localEulerAngles;
            reStartRot = re.localEulerAngles;
        }
        sane = true;
    }

    void OnDrawGizmosSelected() {
        Camera cam = Camera.current;
        Vector3 cameraPosition = transform.position + new Vector3(0f,1.8f,0f);
        Vector3 forward = Vector3.zero;
        if (cam == null) {
            cam = Camera.main;
        }
        if (cam != null) {
            cameraPosition = cam.transform.position;
            forward = cam.transform.forward*5f;
        }
        if (!sane) {
            Gizmos.color = Color.red;
            Handles.Label (cameraPosition + forward + new Vector3(0,2,0), "An error was detected! Check the console log for details...");
            return;
        }
        Gizmos.color = Color.yellow;
        if (warn) {
            Handles.Label (cameraPosition + forward +  new Vector3(0,2,0), "A warning was detected! Check the console log for details...");
        }
        Gizmos.DrawSphere(lookPos, 0.1f);
        if (le != null && re != null) {
            Gizmos.DrawLine (le.position, lookPos);
            Gizmos.DrawLine (re.position, lookPos);
        }
        Gizmos.color = Color.yellow;
    }

    // Update is called once per frame
    void OnAnimatorIK(int layerIndex) {
        if (!sane) {
            return;
        }
        if (!TestEyes) {
            a.SetLookAtWeight (0f);
            return;
        }
        if (eyesSane) {
            a.SetLookAtWeight (1.0f, 0.0f, 0.3f, 1.0f);
        } else {
            a.SetLookAtWeight (1.0f, 0.3f, 1.0f, 0.0f);
        }
        a.SetLookAtPosition (lookPos);
    }

    void Update () {
        if (!sane) {
            return;
        }
        if (a != null && TestWalking) {
            a.SetFloat ("MovementX", Mathf.Sin (Time.time / 1.8f));
            a.SetFloat ("MovementZ", Mathf.Cos (Time.time / 2.2f));
        } else {
            a.SetFloat ("MovementX", 0);
            a.SetFloat ("MovementZ", 0);
        }
        // visually test blinking and lowering lids.
        if (eyesSane && TestEyes && eyeMesh != null && blinkl == 0 && blinkr == 1 && lowerlidl == 2 && lowerlidr == 3) {
            eyeMesh.SetBlendShapeWeight (blinkl, Mathf.Clamp ((Mathf.Sin (Time.time) - 0.99f) * 10000f, 0, 100f));
            eyeMesh.SetBlendShapeWeight (blinkr, Mathf.Clamp ((Mathf.Sin (Time.time) - 0.99f) * 10000f, 0, 100f));
            eyeMesh.SetBlendShapeWeight (lowerlidl, (Mathf.Sin (Time.time/2f)*100f));
            eyeMesh.SetBlendShapeWeight (lowerlidr, (Mathf.Sin (Time.time/2f)*100f));
        }

        // If we have a mesh that has visemes. Rotate through each one to test them visually.
        if (m != null && TestLipSync && d.lipSync == VRCSDK2.VRC_AvatarDescriptor.LipSyncStyle.VisemeBlendShape) {
            if (waitTimer <= 0) {
                waitTimer = waitTime;
                m.SetBlendShapeWeight (m.sharedMesh.GetBlendShapeIndex (d.VisemeBlendShapes [curindex++]), 0f);
                curindex = curindex % d.VisemeBlendShapes.Length;
            } else {
                m.SetBlendShapeWeight (m.sharedMesh.GetBlendShapeIndex (d.VisemeBlendShapes [curindex]), ((waitTime - waitTimer) / waitTime) * 100f);
                waitTimer -= Time.deltaTime;
            }
        // Or just flex the one over and over
        } else if (m != null && TestLipSync && d.lipSync == VRCSDK2.VRC_AvatarDescriptor.LipSyncStyle.JawFlapBlendShape) {
            m.SetBlendShapeWeight (m.sharedMesh.GetBlendShapeIndex (d.MouthOpenBlendShapeName), Mathf.Sin (Time.time) * 100f);
        } // Or we test the jaw flap bone, which happens in LateUpdate...
        // Set the look target position as something random around the character.
        Transform head =  a.GetBoneTransform (HumanBodyBones.Head);
        lookPos = head.position + Vector3.forward * Mathf.Sin (Time.time * 1f)*5f + Vector3.up * Mathf.Sin (Time.time * 0.25f)*5f + Vector3.right * Mathf.Cos (Time.time * 1f)*5f;
    }

    void LateUpdate() {
        if (!sane) {
            return;
        }
        if (TestLipSync && d.lipSync == VRCSDK2.VRC_AvatarDescriptor.LipSyncStyle.JawFlapBone) {
            d.lipSyncJawBone.localEulerAngles = jawStartRot + new Vector3 (0f, 0f, Mathf.Sin (Time.time) * 30f);
        }
        if (!eyesSane || !TestEyes) {
            return;
        }
        if (le != null) {
            /*Vector3 rot = le.localEulerAngles - leStartRot;
            while (rot.x > 180f) {
                rot.x -= 360f;
            }
            while (rot.x <= -180f) {
                rot.x += 360f;
            }
            while (rot.y > 180f) {
                rot.y -= 360f;
            }
            while (rot.y <= -180f) {
                rot.y += 360f;
            }
            while (rot.z > 180f) {
                rot.z -= 360f;
            }
            while (rot.z <= -180f) {
                rot.z += 360f;
            }
            // FIXME: Not sure why these have to be flipped around, might be specific to the orientation of the eye bone or something. Testing needed...
            rot.x = Mathf.Clamp (rot.x, -leLimit.max.y, -leLimit.min.y);
            rot.y = Mathf.Clamp (rot.y, -leLimit.max.x, -leLimit.min.x);
            rot.z = Mathf.Clamp (rot.z, -leLimit.max.z, -leLimit.min.z);
            le.localEulerAngles = rot + leStartRot;*/
        }

        if (re != null) {
            /*Vector3 rot = re.localEulerAngles - reStartRot;
            while (rot.x > 180f) {
                rot.x -= 360f;
            }
            while (rot.x <= -180f) {
                rot.x += 360f;
            }
            while (rot.y > 180f) {
                rot.y -= 360f;
            }
            while (rot.y <= -180f) {
                rot.y += 360f;
            }
            while (rot.z > 180f) {
                rot.z -= 360f;
            }
            while (rot.z <= -180f) {
                rot.z += 360f;
            }
            // FIXME: Not sure why these have to be flipped around, might be specific to the orientation of the eye bone or something. Testing needed...
            rot.x = Mathf.Clamp (rot.x, -reLimit.max.y, -reLimit.min.y);
            rot.y = Mathf.Clamp (rot.y, -reLimit.max.x, -reLimit.min.x);
            rot.z = Mathf.Clamp (rot.z, -reLimit.max.z, -reLimit.min.z);
            re.localEulerAngles = rot + reStartRot;*/
        }
    }

    public bool GetHumanDescription(string target, ref HumanDescription des) {
        if (target != null) {
            string[] assets = AssetDatabase.FindAssets (target, null);
            if (assets.Length <= 0) {
                return false;
            }
            // Figure out which one has the rig
            foreach (string guid in assets) {
                AssetImporter importer = AssetImporter.GetAtPath (AssetDatabase.GUIDToAssetPath (guid));
                if (importer != null) {
                    ModelImporter modelImporter = importer as ModelImporter;
                    if (modelImporter != null) {
                        des = modelImporter.humanDescription;
                        return true;
                    }
                }
            }
        }
        return false;
    }
}
#endif

Doesn't work in the current version of the avatar SDK :(

Assets\Editor\VRChatHelper.cs(41,13): error CS0234: The type or namespace name 'VRC_AvatarDescriptor' does not exist in the namespace 'VRCSDK2' (are you missing an assembly reference?)