Finding position of button in guilayout grid

How do I find the exact position of a button in this GUILayout grid: var aGrid:Card[,] = new Card[4,4];

 var cols:int = 4;
 
 var rows:int = 4; 
 
  
 
 function BuildGrid()
 
 {
 
     GUILayout.BeginVertical();
 
     GUILayout.FlexibleSpace();
 
     for(var i:int=0; i<rows; i++)
 
     {
 
         GUILayout.BeginHorizontal();
 
         GUILayout.FlexibleSpace();
 
         for(var j:int=0; j<cols; j++)
 
         {
 
             var card:Card = aGrid[i,j];     
 
             
 
         if(card.isFaceUp)
 
         {
 
                 img = card.img;
 
         }else{
 
                 img = "faceup";
 
         }
 
       } 
 
         
 
         GUI.enabled = !card.isMatched;
 
                 
 
       GUILayout.Button(Resources.Load(img), GUILayout.Width(sixthOfScreenW), GUILayout.Height(sixthOfScreenH))
 
       
 
     GUI.enabled = true;
 
     }   
 
     GUILayout.FlexibleSpace();
 
     GUILayout.EndHorizontal();   
 
   }
 
   GUILayout.FlexibleSpace();
 
   GUILayout.EndVertical();
 
 }
 
  
 
 //This is the Card class
 
 class Card extends System.Object{
 
     var isFaceUp:boolean = false;
 
     var isMatched:boolean = false;
 
     var img:String;
 
     var id:int;
 
     
 
     function Card(img:String, id:int){
 
         this.img = img;
 
         this.id = id;
 
     }
 
 }