I use two types of thermometers.
1) the indoor/outdoor digital thermometers you can buy in hardware stores. They have a long wire with a probe on the end for the "outdoor" reading, and the main unit gives the "indoor" reading. I place the main unit at the cooler end of the cage and the probe in the basking spot. This means I constantly monitor both ends of the thermal gradient. They also store maximum and minimum temperatures which is nice to know how hot it got during the day when you weren't there or during the middle of the night for example. The main problem with these is that sooner or later they get pooped on and cleaning them is a pain.
2) I also have a laser temperature gun. This gives an instaneous reading for whatever surface you point it at. It is very quick/easy to use, and gives readings for places it might be difficult to place a probe. However, you can't get the max./min. information with these.
Skye