I keep a hard start in my tool bag. I remove the relay and connect the hard start up t the compressor. The HS has a cord on it. I plug that into an outlet and check the amp draw. I listen to the compressor watching the ammeter needle. Most of the time the ammeter will jump to around 6 amps then drop to half amp or a little better if the compressor is good. I try to replace the exact relay if I can but if I can't I just install a hard start device.