// 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);
    }

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void Update() {
        RaycastHit hit;
        if (!Physics.Raycast(camera.ScreenPointToRay(Input.mousePosition), out hit))
            return;
        
        MeshCollider meshCollider = hit.collider as MeshCollider;
        if (meshCollider == null || meshCollider.sharedMesh == null)
            return;
        
        Mesh mesh = meshCollider.sharedMesh;
        Vector3[] vertices = mesh.vertices;
        int[] triangles = mesh.triangles;
        Vector3 p0 = vertices[triangles[hit.triangleIndex * 3 + 0]];
        Vector3 p1 = vertices[triangles[hit.triangleIndex * 3 + 1]];
        Vector3 p2 = vertices[triangles[hit.triangleIndex * 3 + 2]];
        Transform hitTransform = hit.collider.transform;
        p0 = hitTransform.TransformPoint(p0);
        p1 = hitTransform.TransformPoint(p1);
        p2 = hitTransform.TransformPoint(p2);
        Debug.DrawLine(p0, p1);
        Debug.DrawLine(p1, p2);
        Debug.DrawLine(p2, p0);
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def Update():
        hit as RaycastHit
        if not Physics.Raycast(camera.ScreenPointToRay(Input.mousePosition), hit):
            return
        meshCollider as MeshCollider = hit.collider
        if (meshCollider == null) or (meshCollider.sharedMesh == null):
            return
        mesh as Mesh = meshCollider.sharedMesh
        vertices as (Vector3) = mesh.vertices
        triangles as (int) = mesh.triangles
        p0 as Vector3 = vertices[triangles[((hit.triangleIndex * 3) + 0)]]
        p1 as Vector3 = vertices[triangles[((hit.triangleIndex * 3) + 1)]]
        p2 as Vector3 = vertices[triangles[((hit.triangleIndex * 3) + 2)]]
        hitTransform as Transform = hit.collider.transform
        p0 = hitTransform.TransformPoint(p0)
        p1 = hitTransform.TransformPoint(p1)
        p2 = hitTransform.TransformPoint(p2)
        Debug.DrawLine(p0, p1)
        Debug.DrawLine(p1, p2)
        Debug.DrawLine(p2, p0)