try this:
http://imakewebthings.github.com/jquery-waypoints
https://github.com/brandonaaron/jquery-mousewheel
the second one is probably the one you want. You can check for the wheel events then check the mouse cursor position. or something like that. However, you need to consider people will access your page with tablets etc, so really scrollwheel events depending on "mouse position" is actually not a great idea.