Many people just hang a thermometer in one of the vent holes. Once you get used to any particular method it doesn't much matter, though. Keep doing it the same way and you'll soon learn what to expect for cook times, etc.
I would invest in a probe thermometer you can place at grate level. My lid mounted thermometer is anywhere from 15-30 degrees off at all times! If you are buying a quality thermometer a probe isn't much more expensive.