You can get a therometer and RH (relative humidity) meter combo. Not that expensive. You should be able to get one for like 20 bucks or so.

See example:
Amazon.com: Timex TX5170 Indoor/Outdoor Thermometer with Indoor Hygrometer and Clock: Home & Garden
hbbusa28 Reviewed by hbbusa28 on . Humidity question I've read a few threads from Zander about the benefits of humidity control during your grow but how do you check what your humidity level is. Is there some kind of meter I'm not aware of that can do this. Rating: 5