Graeme Wright has a section in his book about this. There's three methods he recommends, the first two involve holding the case on you bare hands - the case should not get hot enough to burn your fingers. The case only needs to show some level of discolouration and not turn cherry red - this would be way too hot for the brass (and fingers).

The third involves the use of temperature sensitive crayons or lacquers (used in welding) and holding the case in a slow speed drill, but only to rotate it for even coverage. The crayons or lacquers change colour at the right temperature.

I can't go into details for risk of breach of copyright, besides, he's a good guy and I don't want to upset him either.