Check if is destroyed

I have this script...

 #pragma strict
 var pnt_1 : UI.Image;
 var CUBE_1 : GameObject;
 
 function Update () {
     if (CUBE_1) {
         pnt_1.enabled = true;
     }
 }

And it check if is object CUBE_1 existing. I want to check it when will be this object destroyed because it will chceck it when game start...