No, just because some snakes will move when its too hot, doesn't mean all of them will. Many will sit there without realizing they are being cooked alive. Thats why you need something like a temp gun, to measure how hot the heat pad actually is, how hot the glass is, and how hot the substrate they are basking on is.
Before I would take the heat pad up to 100 because the substrate wouldn't get hot enough below that. But my BP likes to dig under the thin layer of substrate. He would then be touching the glass that is 100 degree's. So a good rule of thumb is to make sure that glass is not above 95 degrees.