A thermometer will only tell you how warm it is the enclosure. For humidity, try using a humidity gauge.
There's a helpful thread about humidity here (*clicky*), in the Enclosures & Housing forum. Remember to check the appropriate forums, before posting a new topic. Most often, you'll find the information you're looking for.
You can buy a digital thermometer/humidity guage at Wal-Mart for approx. $15. They're pretty accurate and come with double sided tape so you can stick it to your cage wall.