如何判断我的应用程序中的任何表单是否在前台

I'd like my compact framework application to show a notification bubble ONLY when it's running "in the background". The application has several forms which may or may not be visible at any time. I'd like the notification handler (which runs on background separate thread) to be able to tell if any form from my application is currently visible on the screen.I'd like my compact framework application to sh