Longer run time
The first sign. If it used to run 20 seconds and now runs 45, that's a clear signal.
Running while parked
A compressor shouldn't run with the car off except rarely. If it becomes regular, you have a leak.
A different or louder noise
Metallic scraping means the piston ring is worn.
Slow rise after start-up
The car taking noticeably longer to reach normal height.
Intermittent suspension messages
The system logging that pressure wasn't reached within the expected time.
Uneven rise
One side coming up while the other lags.
Catch it in the weakening stage and a rebuild kit may be all you need. Wait until it burns out and you're paying for a new compressor plus the original leak repair — and inspection here is free.
Related service: Air Suspension Compressor & Pumps