As a start, see if the fuel cap will twist after it's locked down. If it moves, it's too loose and needs adjustment. That should address the main O-ring on the standard Van's fuel caps--but it's possible the smaller O-ring on the center shaft could still leak.
As far as the vent system goes, slip a piece of 3/8" vinyl tube over the vent fitting on the bottom of the aircraft and blow on the tube with the fuel cap off. With a helper positioned at the open fuel cap, listen for air coming out. If they can hear air, then you've confirmed the vent system is venting. Blow again with the fuel cap on and locked down and there should be no air escaping. If it is leaking and the big O-ring was tight, it could be leaking past the O-ring on the center shaft. You could spray a TINY amount of soapy water on the cap and see if bubbles form, but there's less risk of water intrusion if you just listen.
If you don't want anyone to see you blowing on your fuel tanks or if a helper isn't readily available, you can use a longer piece of vinyl tubing and listen at the cap yourself. Not sure if that addresses your concern, but hopefully it will.