For any uniflow,that is mounted already, hold the plane pointed nose up. Start filling thye tank through the pickup tube. Put a stopper over one of the other tubes. If you plugged the Vent, fuel will almost immediately start comming out the uniflow vent, since it is right next to the pickup at the (now bottom) of the tank. If you get nearly a full load of fuel into the tank before any fuel comes out, then you plugged the uniflow and the fuel runs out the vent (now at the top end of the tank.
Curt