function IsMyWindow: Boolean;
var
 WndFound: Boolean;
 rslt: boolean;

 function AnalyzeWnd(CurHandle: THandle; CurrentProcessId: LParam): Boolean; stdcall;
 var
   WndProcId: Cardinal;
   Buffer : array[0..255] of Char;
 begin
   Result := True;
   GetWindowThreadProcessId(CurHandle, WndProcId);

   if LParam(WndProcId) = CurrentProcessId then
   begin
     FillChar(Buffer, 256, 0);
     GetClassName(CurHandle, @Buffer[0], 255);

     WriteLog('wnd search: ' + Buffer);

     if (Buffer = 'MyWindow') then
     begin
       WriteLog('MyWindow found');
       WndFound := True;
       WriteLog('WndFound = ' + BoolToStr(WndFound, True));
       //stop Enum
       Result := False;
     end;
   end;
 end;

begin
 WndFound := False;

 rslt := EnumWindows(@AnalyzeWnd, GetCurrentProcessId);

 WriteLog('done, WndFound = ' + BoolToStr(WndFound, True) + ' EnumRslt = ' + BoolToStr(rslt, True));

 Result := WndFound;
end;

wnd search: MyWindow
MyWindow found
WndFound = True
done, WndFound = False EnumRslt = False