I would like to know how to recognize damaged brushless motors on a drone? What are the most common symptoms?
There are many signs that a brushless motor is broken or getting worn out. Something that can happen to motors that have been used for a long time is their bearings wear out. If you notice that the bell of a motor feels loose or the motor is excessively noisy or has a grinding sound, it is likely worn-out bearings. You can either replace the bearings or buy a new motor.
Another thing that happens is the magnets can demagnetize over time if the motors get hot. A way to know if this has happened is to run the motor with no load. If the RPMs are unusually high, the magnets are likely losing their strength. In this case, you need a new motor.