I have always tuned my car by temp but it is a bit of a PITA to get a "good" head reading on a heli. Putting you fingers on the backplate is the popular "tried and true" method. (Pretty hot but it not fry your fingers).

You should be able to hold them on there for a few seconds without having to move from pain. If I remember you do not want the motor over 240F at the head.