[TenTec] When is "hot" too hot?

[nz7c at cox.net]

Please bear with this question.....as a rule of thumb from you guys that have been around radios all your lives, how do you know when your rig is running too hot? Don't laugh. For example, as long as you can hold your hand on the fins of the heat sink is it OK - even if it feels pretty toasty? Or, if you touch the fins and burn the crud out of yourself, is it time to quit? You get the idea. Just wanted a little input :-)
73/Tim NZ7C