The accurite thermometer / hygrometer that bcr mentioned will tell you the temps and the humidity in the tank. I got the indoor / outdoor one and use the long probe on the hot side directly over the hotspot (I hot glued it in place), then place the unit on the oppisite end of the tank and it should tell you the hot side temp, the cool side temp, and the humidity.

The herpstat probe is just to control how much power goes to the heat mat... It shouldn't be used to read humidity.

Oh, and btw, never use tape in the enclosure to secure wires or anything... which is why I used the hotglue to secure the thermometer probe and not tape. Apparently snakes can get stuck to tape very easily.