Оформил: DeeCo
Автор: http://www.swissdelphicenter.ch

{ 

  You would have to iterate over all windows above yours in Z-order and 
  check for each window you find if it has the WS_EX_TOPMOST exstyle set 
  and is visible. 
  If it has, you have to get its window rectangle (GetWindowRect) and test 
  if that overlaps your window. 

  Example: 

}

 procedure TForm1.Button1Click(Sender: TObject);
 var
   wnd: HWND;

   function IsTopMost(wnd: HWND): Boolean;
   begin
     Result := (GetWindowLong(wnd, GWL_EXSTYLE) and WS_EX_TOPMOST) <> 0;
   end;

   procedure logWindowInfo(wnd: HWND);
   const
     visString: array[Boolean] of string = ('not ', '');
   var
     buffer: array[0..256] of Char;
     r:      TRect;
   begin
     if wnd = 0 then Exit;
     GetClassName(wnd, buffer, SizeOf(buffer));
     with Memo1.Lines do
     begin
       Add(Format(' Window of class %s ', [buffer]));
       GetWindowRect(wnd, r);
       Add(Format(' at (%d,%d):(%d,%d)', [r.Left, r.Top, r.Right, r.Bottom]));
       Add(Format(' Window is %svisible', [visString[IsWindowVisible(wnd)]]));
       Add(Format(' Window is %stopmost', [visString[IsTopmost(wnd)]]));
     end;
   end;

    begin
   Memo1.Clear;
   wnd := Handle;
   repeat
     wnd := GetNextWindow(wnd, GW_HWNDPREV);
     LogWindowInfo(wnd);
   until wnd = 0;
   Memo1.Lines.Add('End log.');
 end;