RaycastHit.triangleIndex  

// Attach this script to a camera and it will
// draw a debug line triangle triangle
// at the triangle where you place the mouse.
function Update () {
// Only if we hit something, do we continue
var hit : RaycastHit;
if (!Physics.Raycast (camera.ScreenPointToRay(Input.mousePosition), hit))
return;

// Just in case, also make sure the collider also has a renderer
// material and texture
var meshCollider = hit.collider as MeshCollider;
if (meshCollider == null || meshCollider.sharedMesh == null)
return;

var mesh : Mesh = meshCollider.sharedMesh;
var vertices = mesh.vertices;
var triangles = mesh.triangles;

// Extract local space vertices that were hit
var p0 = vertices[triangles[hit.triangleIndex * 3 + 0]];
var p1 = vertices[triangles[hit.triangleIndex * 3 + 1]];
var p2 = vertices[triangles[hit.triangleIndex * 3 + 2]];

// Transform local space vertices to world space
var hitTransform : Transform = hit.collider.transform;
p0 = hitTransform.TransformPoint(p0);
p1 = hitTransform.TransformPoint(p1);
p2 = hitTransform.TransformPoint(p2);

// Display with Debug.DrawLine
Debug.DrawLine(p0, p1);
Debug.DrawLine(p1, p2);
Debug.DrawLine(p2, p0);
}