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;

public class Example : MonoBehaviour
{
    // Is a collision between two objects with different roots?
    void OnCollisionEnter(Collision collision)
    {
        if (collision.transform.root != transform.root)
        {
            print("The colliding objects are not in the same hierarchy");
        }
    }
}