All of the three options (Run the engine at a fast idle, Step on and off the brake to reduce tank pressure, and Monitor the air pressure gauge) are correct. To check your air compressor governor cut-in and cut-out pressures you should run the engine at a fast idle and monitor the air pressure gauge to see if the air governor cuts out at the manufacturer's specified pressure (typically about 125 psi). With the engine idling, you should step on and off the brake to reduce tank pressure and see if the governor cuts in at the manufacturer's specified pressure (typically about 100 psi).