An inexpensive garden thermometer will suffice. Correct temperatures are critically important and keepers are notoriously bad at estimating them. There are few better investments for a keeper than good, efficient equipment to monitor temperatures. Electrically-heated rocks can be very dangerous to captive snakes, and we do not recommend this type of heating element for snakes, even though they are marketed by the pet industry as being ideal for snakes. The problem lies with the fact that, while most snakes appear to have excellent abilities to identify dangerous radiant temperatures on their dorsal surfaces, at least some snakes apparently have problems detecting dangerous temperatures through their bellies.
Estimation is dangerous.
But when estimation is the only means available, one way to measure the potential of a heated rock is to hold it tightly in the hand for five minutes. A heated rock should feel warm or even very warm to the hand of a keeper, but if the rock is a safe temperature, then it can be tightly held in the hand for five minutes without becoming uncomfortably hot to the hand. As is true for many snakes, Trans-Pecos ratsnakes can better tolerate temperature extremes if they do not have food in their digestive systems. Trans-Pecos ratsnakes do seem to be able to eat and digest at relatively cool temperatures in the low 70s. However, we feel this may pose some hazard to them, particularly if the meals are large.