Agreeing with recording readings and then watching for higher than your norms. Also consider shooting the drums if possible. Low drum temp can indicate under performing brake and high temps can indicate brake drag.
It's a very useful tool.
The only documented number I can offer comes from the Dill owners manual. It alarms at internal temp over 176 deg F.
2005 Dodge Ram 2500 5.9L 6 Speed
16" Michelins, Hi Spec Wheels, Max Brake, Carslile Actuator, Equal-i-zer, Dill TPMS. Campfire cook.