A lot of people use these:
http://www.heartfeltindustries.com/p...at=Hygrometers
I use the circle ones, but I have seen alot of people using the rectangle ones. Both can be calibrated and for an extra couple bucks you can order them with a calibration pack. The calibration packs are much better than the "salt-test" because if you add too much water or too little water it can be off.