How did you implement .VisibleOnScreen property?

I am very curious... I am working on a management app here, and I have been looking all over at how I could possibly figure out if a form is in clear view or hidden behind another, and it seems there is no real way to figure that out easily in either .Net or Win32. I thought of figuring out which of all the windows are occupying the same rectangle on screen, then checking which has higher Z-Index... but the z-index of a form is no longer exposed as a number anywhere, you can only .SendToBack and .BringToFront, which is not what I want to do... I know you wouldn't  share your source for it, but I would greatly appreciate the friendly gesture of pushing me in the right direction.



Thank you!