I suspect this is probably not possible due to browser security, however I have a form at the end of which is an iframe displaying our terms and conditions page. Below is a checkbox for agreeing to these terms and conditions.
Other sites I have seen require you to scroll down in the terms and conditions before the checkbox is active but i suspect this is done with a textarea or scrollable div rather than an iframe.
As such is anyone aware of how you can spot when an iframe has scrolled in javascript or jQuery so I can enable the checkbox once it has been scrolled.
Following code in your embedded document should do it.