Transform.root

Description

Returns the topmost transform in the hierarchy.

(This never returns null, if this Transform doesn't have a parent it returns itself.)

using UnityEngine;
using System.Collections;

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