Я не пробовал:
{Wnd = your app. handle}
if IsWindowVisible(Wnd)
and
( (GetWindowLong(Wnd, GWL_HWNDPARENT) = 0) or (HWND(GetWindowLong(Wnd, GWL_HWNDPARENT)) = GetDesktopWindow) )
and
((GetWindowLong(Wnd, GWL_EXSTYLE) and WS_EX_TOOLWINDOW) = 0)
then
// your application is visible on taskbar