[FrankJ]

Yes it possible to blow them. Harder to tell if they are partially degraded. Many teams use new motors in their competition bots for that reason.

One possibility would be to use a good ohm meter & slowly turn the motor and watch the coil resistance & compare it to a new motor. That will only show a badly degraded motor though. like one with open windings.

Another possibility would be to run it with a known load & compare its speed & current draw against a new motor.