[hatzie]

Your tool should cycle the vent closed and the purge open to pull a vacuum on the fuel tank to allow you to look at the tank pressure.
I can individually command the purge and vent valves with the Tech 2 and command an evap test with a specific purge duty cycle % to watch the effect on the tank pressure PID.
If your tool can command the vent it shroud be able to command the purge valve as well. It should also let you view the tank pressure data PID. The evap test should allow you to specify a purge duty cycle but 100% is good enough if you can view the tank pressure.

You can test the purge valve for a gross leak by checking for vacuum through the valve when it's supposed to be closed with the electrical plug disconnected... there should be zero vacuum with it unplugged and the engine running. You need to command it open to check for vacuum when it's open.

If you suspect a vent blockage... command the purge to 50% and vent open. The tank pressure shouldn't drop much since there should be a path through the evap can and vent valve to the open air.
If you overfilled the tank and saturated the carbon in the canister with fuel it may not purge very well if at all.
Mud daubers and spiders like to make nests in the vent intake hoses for some reason. Check there before changing parts.

If you drive in dusty conditions or have critters making a home in the vent hose opening you can either run an SAEJ30R7 hose up to the clean side of the air filter or install a 19208702 vent filter from the GMT900 trucks on the vent hose.