I own a brushed micro quadcopter, more commonly known as a tiny whoop. I've read that brushed motors in general, and especially those used in such quadcopters, have a limited lifetime and that you should replace them when they near the end of that lifetime.
However, as far as I can tell, there are no obvious signs that the motors have become too old to fly, as they don't break but instead their performance decreases over time. There's a maximum total runtime in hours listed for the motors, but I can't imagine any way to track how many hours the current set has been flown for besides writing down the length of each flight, which would be extremely tedious.
So how do I figure out whether my motors are worn out and should be replaced, or are still okay to fly with?