// Is a collision between two objects in different hierarchies?
function OnCollisionEnter(collision : Collision) {
    if ( collision.transform.root != transform.root ) {
        print("The colliding objects are not in the same hierarchy");
    }
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void OnCollisionEnter(Collision collision) {
        if (collision.transform.root != transform.root)
            print("The colliding objects are not in the same hierarchy");
        
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def OnCollisionEnter(collision as Collision):
        if collision.transform.root != transform.root:
            print('The colliding objects are not in the same hierarchy')