Your finger is not a thermometer.
Any analog circuit designer who doesn't work with lethal voltages will disagree.
For most people, the threshold between holding your finger on something vs. saying "ouch" and pulling away (or having to force yourself to touch it if you're trying to be manly) is 50 degrees C. 50 degrees C also happens to be about the maximum temperature you want your circuit to be in a normal room, at least if you want the thing to work when it's in an engine compartment of a car, or sitting on a runway in a desert somewhere, waiting to help send love notes to your nation's enemies.
That's also not a bad baseline for the case temperature of a motor right after a flight. Guaranteed, if the outer case (which is in the airflow and isn't the part that generates heat) is over 50 degrees C, the motor innards are significantly hotter.
Having said that -- Horror Fright sells cheap IR thermometers that work pretty good -- you just need to make sure you have a clear shot at the motor.
Here's what I had to do to cool down the motor on my slime to 'lectric conversion, in addition to making sure there was an exit path for the air. It's not pretty, but an ugly airplane flying is better than a pretty one on the ground: