How to detect the load event of iframe which is inside a html scope from the outside of iframe(parent html)

a few notes:

• 1.i can not modify the iframe content to add the onload= function
• 2.the iframe has an id.
• 3.the iframe has no callback to its parent html page.

• 4.I use the prototype ,not jquery, for some old reasons.

  i wanna do something when the iframe load finished. but, i have tried the

  document.observe('dom:loaded',function() {

      alert($('iframe-id'));  >> **IS NULL**
  

  })

and

Event.observe(window,'load',function(){..
alert($('iframe-id'));  >> **IS NULL开发者_StackOverflow社区 STILL**

...})

Did i miss some methods in prototype or javascript?

Thanks.