In my experience they actually start to smell before they are fried.
In some cases the plastic fans melt. Once the fan melts you are in trouble because thats what was cooling the motor.
We have had the 2003-style motor smoke, but then still be usable afterwards.
What we use as the F-P test, and what has worked so far this year where we are running the "6V" motors in our drive system - is to touch the motor: if it is almost too hot to touch, then you should stop running it and let it cool. Don't let the motor get too hot to touch - and of course don't stall the darn thing - and you might be able to make it last.
I don't recommend "thermal-shock-in-a-can" either
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Ken