procedure TForm1.Timer1Timer(Sender: TObject); 
var 
  Dir: string; 
begin 
  Dir:=Edit1.Text; 
  if FindFirstChangeNotification( 
       PChar(Dir), 
       False, 
       FILE_NOTIFY_CHANGE_DIR_NAME)<>INVALID_HANDLE_VALUE then 
    Label1.Caption:=Dir+' directory presents' 
  else 
    Label1.Caption:=Dir+' directory absents'; 
end;

Zdroj: http://www.delphitricks.com/source-code/files/detect_changings_of_a_directory.html