Get a meat thermometer and stick the end in the vapor path. You'd have to leave it there for quite some time though to let it come up to temp. Also, if it is not a forced air vaporizer, you would also have to keep drawing air past it to get an accurate reading. Also, depending on how technically savy you are, a K type thermocouple hooked up to a volt meter would have a much shorter response time and give you a more accurate reading. No matter how you measure though be sure your temperature probe is not physically contacting the metal/glass surface as this will give you a false/high reading.
I would stay away from infrared guns in this case as they would likely give you a surface temperature and it may be hard to get the emissivity setting correct.