RaycastHit.triangleIndex

Description

The index of the triangle that was hit.

Triangle index is only valid if the collider that was hit is a MeshCollider.

// This script draws a debug line around mesh triangles
// as you move the mouse over them.
using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour
{
    Camera cam;

    void Start()
    {
        cam = GetComponent<Camera>();
    }

    void Update()
    {
        RaycastHit hit;
        if (!Physics.Raycast(cam.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);
    }
}

Additional resources: Physics.Raycast, Physics.Linecast, Physics.RaycastAll.