Gas tank cap venting?

[History]

I assume the gas tank cap is designed to vent from internal pressure and as the gas heats up it expands,,,,,,,,,. I also assume it's designed to let air in to equalize pressure as the gas gets cold and condenses? How could I check my cap before I put gasoline in as my tank is bone dry at the moment? Should I be able to hear air escaping if I pressurized the tank slightly?

I'd like to not have to wonder later.