Get number pixels form Scrolled

Is there a way of finding out how far a user may have scrolled down a form.

For example if the form has a height of 1500 pixels and lets say a tab panel has a height of 1000 pixels then they can scroll through a further 500 pixels.

I’d like to determine what part of the form is actually being seen at any time.

I’d like to do this for web client.