keep in mind that I highly doubt the speed of the motors would all be exactly the same, even when hovering.
The rpm of each motor is how drones do everything, yaw - pitch - climb etc.. even in a hover, the drone is holding position and the slightest breeze or even ground effect, would require slight corrections making the rpms different for each motor.
If you're looking to make sure each motor is doing ok, I've read the best way to do that is to get one of those laser/ir temperature measuring guns and check to make the temperature of each motor is similar (again, they won't be exactly the same). Note the temperature of each motor in a calm location hovering just above the ground. On a similar day (ambient temperature wise), check again and if the temperature of any of the motors is a lot different, then you know you MAY have a problem.
Again, not a sure indicator, but higher operating temperature is the first indication of bad bearing. And bearings are the only real failure point (aside for wire connections) that can go bad on brushless motors.