The best way to check the CHECK Valve is to disconnect the compressor side of the check valve after filling the air tank. Then use soapy water to see if it is bleeding back from the check valve.
If it is, take it out of the system and physically look at it and test it by pushing air through, the air should only pass in one direction and not the other. If it does leak don’t try to fix it just replace it, its safer.
If it works OK put it back in the system and cross that of the list.
If you think your air system is leaking remember to check that the CHECK valve is working properly. It is possible for a check valve to foul up. It can be grit in the air system or failing to drain the air tank regularly or maybe its worn out and rusty!
A check valve is normally located close to, or at the Air Tank between the tank and the Compressor and it is essentially a one way valve that lets air flow from the compressor to the tank but NOT the other way. Its purpose it to take the pressure off the compressor as it pumps eliminating dead hitting and damaging the compressor piston or crank.
Compressors don’t like to be started under pressure.
Need a one-way valve? Check Valves are here