There are a few things you can try. First, check the air pressure in the system at the Schrader valve under the seat. On an RTS, there should always be at least ten pounds of pressure. If there is pressure, check again after sitting overnight, to see if it leaks down. It should not. You can check the function of the shock by pushing down sharply on the rear of the Spyder. It should rebound in a single motion, rather than bouncing, and should not make a clunking noise. It is best to do this test with the air suspension set no higher than midway.
If you also want to test the automatic suspension (ACS) function, start the Spyder, put it in gear, then change the settings. Have someone else observe or measure the height at the rear fender. Upon raising the setting, the compressor should run and the rear should rise. Upon lowering the setting, the solenoid valve should release air, and the rear should lower.