Here is yet another small code snippet which allows you to get the order of MovieClips that are positioned on the stage.

var boxes:Array = [box1, box2, box3, box4]; //put the MovieClips into an array

function getOrder():void
{
  boxes.sortOn("x", Array.NUMERIC); //sort array on the x positions from smallest to largest
  trace(boxes); // traces the order of the boxes according to their x positions
}

getOrder();

Source for the demo above: Download