//Find windows that may cover another window.
Var
 hW: HWnd;
 r: TRect;
begin
 hw := Handle;
 While IsWindow(hw) Do Begin
  hw := GetWindow( hw, GW_HWNDPREV );
  If IsWindowVisible(hw) and not IsIconic( hw ) Then Begin
  ... use GetWindowRect( hw, r ) to get candidate windows
  rect and check if it intersects the forms BoundsRects via
  IntersectRect
  End;
 End;
end;