Version: 2017.2
public 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"); } }