Self sealing(capless) fuel fillers

Not really much to check,just insure that the flap at the top of the filler neck tube is in the close position.
Some of the early vehicles had a seal on the inside of aperture flap on the 1/4 panel which where prone to splitting.
From a testing point of view there is no need to stick anything in the filler neck,
On a non testing matter;

The sealing flap at the top of the filler neck shouldn't be forced open as the mechanism further down into the tube which consists of a few compartments may be damaged.

Take the ignition key/card out to unlock the flap within the tube just as if you where putting fuel in,as I think the flap/mechanism will not fully open if the key is still in the ignition or the car is locked.