A little script that draws the direction of mesh normals into the scene window in Unity3D, and also draws little number labels for each of the mesh's vertices. Just add the component to the object containing the mesh. Works with MeshFilter and MeshCollider meshes.

VisualiseMeshNormals.cs

using UnityEditor;
using UnityEngine;

public class VisualiseMeshNormals : MonoBehaviour {
    public enum MeshType { Filter, Collider }
    public MeshType meshType = MeshType.Filter;

    protected MeshFilter meshFilter;
    protected MeshCollider meshCollider;

    protected Mesh mesh {
        get {
            if (meshType == MeshType.Filter && meshFilter == null) {
                meshFilter = GetComponent<MeshFilter>();
            } else if (meshType == MeshType.Collider && meshCollider == null) {
                meshCollider = GetComponent<MeshCollider>();
            }
            return meshType == MeshType.Filter ? 
                meshFilter.mesh : meshCollider.sharedMesh;
        }
    }

    void OnDrawGizmos() {
        for (int i = 0; i < mesh.vertexCount; i++) {
            Gizmos.color = Color.yellow;
            Handles.Label(transform.position + mesh.vertices[i], i.ToString());
            Gizmos.DrawLine(transform.position + mesh.vertices[i], 
                transform.position + mesh.vertices[i] 
                + mesh.normals[i].normalized * 100);
        }
    }
}