Howto check if form is visible in code

Hi

I have a form that updates its controls when some external event is
received.
Because the updates are relatively expensive, I want to update only when
the form, or part of it, is actually visible on the screen.

I have of course tried using the Visible property, but does not change when
my window is covered by another window.
The Activated event is no help either, because I do want to update my
controls when my window is not active, but still visible.

Any ideas?
Thanks.

- Hildo den Breejen