mechanically coupled DC motors

[misterT]

The windings will heat up faster than the heat is able to transfer through a metal case to a sensor.

True.. Temp sensor is too slow to detect over heating due to motor stalling (locking).

Locked rotor current is the industry standard for checking maximum load current.

That is called Stall Current or Starting Current which is measured at nominal voltage.

Another industry standard is maximum load current is usually 3 times the maximum run current listed in the motor tag. If the tag says max run current is 5 amps then locked rotor current should be about 15 amps. You can test it yourself and see. Lock the shaft so it can not turn then turn on the motor with and amp clamp or amp meter in the circuit. Take a quick 5 second reading. These will tell you a lot about the design of the motor. Most good motors will be 3, some manufactures might be about 2.5 to 2.7, cheap Chinese motors are 2.

I have worked mainly with good quality servo motors where all these specs are measured and calculated pretty accurately. No "rule of thumb" numbers.
This is what a good motor spec looks like:


EDIT: Since dmta doesn't have all those "fancy specs", everything that gary350 said is very usefull "rule of thumb" info. We are just talking about two different cases here. I'm talking about small precision servo drives and gary350 is talking about industry workhorses.