WIP - Simple editor script to provide options in dropdown menu when clicking on an asset in the project browser to discover what other assets/scenes reference it.

AssetsMenu_ReferenceChecker.cs

using System.Collections.Generic;
using System.IO;
using System.Linq;
using UnityEditor;
using UnityEngine;

namespace NoiseCrimeStudios.Core.Editor.Asset.Tools
{
    /// <summary>
    /// Editor functions to obtain useful information about Assets in Project Browser.
    /// Right-Click on asset, navigate to 'Reference Checker' then 'Use In Scene', 'Used By Objects' or 'Missing References'.
    /// Based on original code by Matt "Trip" Maker from Monstrous Company :: http://monstro.us
    /// http://unifycommunity.com/wiki/index.php?title=UnityAssetXrefs
    /// </summary>
    public class AssetsMenu_ReferenceChecker
    {
        #region Used In Scene
        [MenuItem("Assets/Reference Checker/Used In Scene", false, 201)]
        private static void SelectSceneUsesOfAsset()
        {
            Object cur = Selection.activeObject;

            // Optionally tell the user what's going on // TODO get only main assets of above
            //	Object[]        sceneObjects    = Object.FindSceneObjectsOfType(typeof(Object)); // Depreceated
            //  Object[]        sceneObjects    = Object.FindObjectsOfType( typeof(Object) );
            Object[] sceneObjects = Object.FindObjectsOfType<Object>();
            List<Object> results = CollectReverseDependenciesInCurrentScene(cur);

            // Debug.Log("Usage: Used In Scene: You have asked which of " + sceneObjects.Length + " assets make use of " + cur.name + ", a " + cur.GetType().ToString() + ".", cur);
            // Debug.Log("Usage: Used In Scene: " + results.Count()  + " uses in scene found for this asset.", cur);
            // printAssetPaths(results);

            Debug.Log("Reference Checker: Used In Scene: " + results.Count() + " / " + sceneObjects.Length + " found for " + cur.name + ", a " + cur.GetType().ToString() + ".", cur);

            foreach ( Object result in results )
            {
#if UNITY_2018_1_OR_NEWER
                Object parentPrefab = PrefabUtility.GetCorrespondingObjectFromSource(result);
#else
                Object parentPrefab     = PrefabUtility.GetPrefabParent( result );
#endif	
                string parentPrefabType = parentPrefab == null ? "null" : parentPrefab.GetType().ToString();

                Debug.Log(result.name + " : " + parentPrefabType, result);
            }

            SetSelection(results);
        }

        private static List<Object> CollectReverseDependenciesInCurrentScene(Object b)
        {
            Object[] sceneObjects = Object.FindObjectsOfType(typeof(Object)); // Object.FindSceneObjectsOfType(typeof(Object));
            return sceneObjects.Where(a => ADependsOnB(a, b)).ToList();
        }
        #endregion


        #region Used By Objects
        [MenuItem("Assets/Reference Checker/Used by Objects", false, 201)]
        private static void FindUsesOfSelectedAssets() { FindUsesOfSelectedAssets(false, false); }

        [MenuItem("Assets/Reference Checker/Used by Objects (Select)", false, 201)]
        private static void FindUsesOfSelectedAssetsHighlight() { FindUsesOfSelectedAssets(true, false); }

        [MenuItem("Assets/Reference Checker/Used by Objects (Itemise)", false, 201)]
        private static void FindUsesOfSelectedAssetsItemised() { FindUsesOfSelectedAssets(false, true); }

        private static void FindUsesOfSelectedAssets(bool highlightResults, bool itemised)
        {
            // Get all selected objects, including folders and folder contents
            Object[] selectedObjectArray = Selection.GetFiltered(typeof(Object), SelectionMode.DeepAssets);

            foreach ( Object selected in selectedObjectArray )
            {
                // Get Asset Path of selected object
                string assetPath = AssetDatabase.GetAssetPath(selected);

                // Check if path is to file or folder.
                if ( !Directory.Exists(assetPath) )
                    SelectUsesOfAsset(selected, highlightResults, itemised);
            }
        }

        private static void SelectUsesOfAsset(Object objectToSearch, bool highlightResults = false, bool itemised = false)
        {
            float startTime = Time.realtimeSinceStartup;
            List<Object> results = CollectReverseDependencies(objectToSearch);

            string title = string.Format("SelectUsesOfAsset: {0} reverse dependencies found for '{1}' in {2:F2} seconds.\n", results.Count(), objectToSearch.name, ( Time.realtimeSinceStartup - startTime ).ToString());

            if ( itemised )
            {
                Debug.Log(title, objectToSearch);

                foreach ( Object obj in results )
                    Debug.Log("   > " + AssetDatabase.GetAssetPath(obj), obj);
            }
            else
            {
                foreach ( Object obj in results )
                    title += AssetDatabase.GetAssetPath(obj) + "\n";

                Debug.Log(title, objectToSearch);
            }

            if ( highlightResults )
                SetSelection(results);
        }

        private static List<Object> CollectReverseDependencies(Object b)
        {
            return AllAssets.Where(a => ADependsOnB(a, b)).ToList();
        }

        private static List<Object> CollectReverseDependencies(Object[] objs)
        {
            List<Object> ret = new List<Object>();
            foreach ( Object obj in objs )
                ret.AddRange(CollectReverseDependencies(obj));
            return ret;
        }
        #endregion


        #region Select Missing References
        [MenuItem("Assets/Reference Checker/Select Missing References", false, 201)]
        private static void SelectMissingReferences()
        {
            float startTime = Time.realtimeSinceStartup;
            List<Object> results = CollectMissingRefs();

            string title = string.Format("SelectMissingReferences: {0} missing references found in {1:F2} seconds.\n", results.Count(), ( Time.realtimeSinceStartup - startTime ).ToString());

            PrintAssetPaths(title, results);
            SetSelection(results);
        }

        private static List<Object> CollectMissingRefs()
        {
            return AllAssets.Where(x => HasMissingRef(x)).ToList();
        }

        private static bool HasMissingRef(Object obj)
        {
            if ( !obj )
                return false;
            Object[] dependencies = EditorUtility.CollectDependencies(new Object[1] { obj });
            foreach ( Object dep in dependencies )
                if ( dep == null )
                    return true;
            return false;
        }

        private static void PrintAssetPaths(string title, List<Object> objs)
        {
            foreach ( Object obj in objs )
                title += AssetDatabase.GetAssetPath(obj) + "\n";

            Debug.Log(title);
        }

        private static void SetSelection(List<Object> results)
        {
            if ( results.Count() > 0 )
                Selection.objects = results.ToArray();
        }
        #endregion


        #region General Methods
        // everything below this line would normally be in other libraries I've made, but I've brought these versions of them into here for simplicity.
        private const string ForwardSlash = "/";
        private const string BackSlash = "\\";

        private static List<Object> AllAssets
        {
            get
            {
                // get every single one of the files in the Assets folder.
                List<FileInfo> files = DirSearch(new DirectoryInfo(Application.dataPath), "*.*");

                // now make them all into Asset references.
                List<Object> assetRefs = new List<Object>();

                foreach ( FileInfo fi in files )
                {
                    if ( fi.Name.StartsWith(".") )
                        continue; // Unity ignores dotfiles.
                    assetRefs.Add(AssetDatabase.LoadMainAssetAtPath(GetRelativeAssetPath(fi.FullName)));
                }

                return assetRefs;
            }
        }

        private static bool ADependsOnB(Object obj, Object selectedObj)
        {
            if ( selectedObj == null )
                return false;
            //optionally, exclude self.
            if ( selectedObj == obj )
                return false;

            Object[] dependencies = EditorUtility.CollectDependencies(new Object[1] { obj });
            if ( dependencies.Length < 2 )
                return false; // if there's only one, it's us.
                              // Debug.Log(obj.name + " has " + dependencies.Length + " dependencies", obj);

            foreach ( Object dep in dependencies )
                if ( dep && ( dep == selectedObj ) )
                    return true;

            return false;
        }

        /// <summary>DataPath uses forward slashes on all platforms now, so replace with Backslash.</summary>
        private static string FixSlashes(string s)
        {
            return s.Replace(BackSlash, ForwardSlash);
        }

        /// <summary>Convert filesystem path to Unity Asset folder relative path so it can work with UnityEditor's built-in commands.</summary>
        private static string GetRelativeAssetPath(string pathName)
        {
            // DataPath uses forward slashes on all platforms now
            return FixSlashes(pathName).Replace(Application.dataPath, "Assets");
        }

        // given a folder and a search filter, return a list of file references
        // (in the unlikely event you have some filesystem arrangement with recursive "hard links", be aware this may not work out well for you)
        private static List<FileInfo> DirSearch(DirectoryInfo d, string searchFor)
        {
            List<FileInfo> founditems = d.GetFiles(searchFor).ToList();

            // Add (by recursing) subdirectory items.
            DirectoryInfo[] dis = d.GetDirectories();

            foreach ( DirectoryInfo di in dis )
                founditems.AddRange(DirSearch(di, searchFor));

            return ( founditems );
        }
        #endregion
    }
}