a thermometer with a probe is ideal. acurite indoor/outdoor thermometers are perfect and at $12 they are pretty inexpensive. they have a probe that you can put right on the substrate. that is the best way to get a very acurite temp reading.

and just a note, the probe for the thermostat being in the tank is not a great idea. the snake could move it off of the UTH and the thermostat will keep the UTH running well past the set temp. a great place to put it is between the UTH and the glass bottom of the tank. then you can monitor it safely, and no worries.