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

procedure TForm1.Button1Click(Sender: TObject);
begin
  if isWindowOnTop(FindWindow('notepad', nil)) then
    Label1.Caption := 'Window ist on the top'
  else
    Label1.Caption := 'Window ist not on the top';
end;