You at least need CO2 coming into the regulator to test it. Attach the regulator to the CO2 tank. Makes sure the working pressure is reading 0 psi. The tank pressure should read 800 psi. Then, close the CO2 tank valve. The tank pressure should still be reading 800 psi. If after couple of days and the pressure is still reading 800 psi, you have no leaks from the regulator to the CO2 tank.
After the test is done, how would you discharge the CO2 inside regulator? Just let it air out?