You probably need to subclass the Taskbar window or constantly "bombard" it with ABM_GETSTATE messages (timer).

http://msdn2.microsoft.com/en-us/library/ms630813.aspx

If subclassing, then probably send ABM_GETSTATE message on each occurrence of WM_NOTIFY | WM_MOVE | WM_POSCHANGED to check the status of the taskbar window.

Or even call API functions like GetWindow, IsWIndowVisible, GetWindowRect with taskbar window handle as input parameter for the same purpose.

This is a guess only; myself I would start digging in this direction.