There used to was to be a crayon like stick
called Tempel stik that would melt at a preset
temp, You would rub the stick on the metal part
and if the part exceeded the temp of the stick it
would melt.  The ideal way to do the procedure was
to put more than one mark, nominal temp, too high
and too low. If the low temp didn't melt you
weren't getting to operating temp. This way you
knew the range of you motor.

    John