I assume that you mean diagnosing OFF the car. The only thing I know would be to turn it by hand to see that it is smooth and put your thumb over the outlet while turning to see if it pulsates with pressure.

Flushing is best done with a flush gun, but many people simply use brake cleaner which is already in a pressurized can. Make SURE, SURE, SURE that you get ALL of the flushing agent out of the system whatever the agent might be.

Flush the compressor with refrigerant oil ONLY.

Good luck,