What kind of thermometer are you using? Digital w/probe ect.
You'll be measuring air temps so you don't want it close to a heat source. Ideally you'll have 2 digital thermometers setup monitoring temps, one on the warm side and another on the cool side, if you have a probe you'll want to attach it to something near the ground. If you affix it to the glass just make sure it's on a side that doesn't have the heat pad attached.