i think there are many ways these motors can fail.
With electrical issues, which will be hard to detect, you may be able to 'smell' a burnt smell if you get close to the pod.
With mechanical issues, you may feel grit or roughness when trying to spin the motors. There can be a bit of a magnetic bump feel as you turn the motor, this is normal. You should spin the motor by hand and notice if it feels different than the others.
Anything that feels gritty or not smooth can be suspect. Also if there is extra shaft up/down or side to side play, this may not be a good sign.
If you operate in an area where sand or fine road dust or grit can get blown into your motors, you could wear them out. Or the magnets could attract fine iron oxide and cause issues.
Flying from a dusty, sandy street or a beach could cause issues like this.
You may also notice that one motor feels hotter than the others, check after each flight. They should all feel about the same temperature, possibly warm, but not so hot that you can't keep your finger on it.
There may be some info in the logs that can help to point a finger, but I'm not sure off the top of my head.
If others have other suggestions, I'm sure they will chime in.