GUILayer.HitTest should help. Something like (attach to the camera):

var gui:GUILayer = GetComponent(GUILayer);
var element = gui.HitTest (Input.mousePosition);
if (element) {
    // hit!
}

Or use OnMouseOver callback (this is not available on Unity iPhone though)

An alternative is to use the OnMouseOver callback (but it's not available on the iPhone, I believe)