One Acu-Rite will do (about $12.00 from Walmart plus a AAA battery). Put the actual unit on the cool side, run the long thin probe wire over to the warm side and tuck the probe inside the warm side hide. I usually run the wire out and back in the enclosure to avoid the snake getting tangled in it. Push a few buttons and you should get....

IN Temp - the temp the unit is reading aka your cool side temp at substrate level
OUT Temp - the temp the probe is reading aka your temp inside the warm side hide at substrate level
HUMIDITY - ambient humidity of the enclosure

Acu-Rite's aren't perfect of course but they are a darned handy and cost effective method of keeping an eye on your enclosure environment.