Get most active Modalpopupextender id

i have a app where have lots of modalpopupextender and some time depending upon navigation, we can have one modalpopup open on top of another modalpopup. so using javascript, how do i get the most active modalpupup extender id?? i can do $find('modalid'), but it will return me object of both modalpopup which are open on top of one another....but i need to know which one is active and which is not active.....