Sort of like the message you get when you try to light something you aren't holding.
If you are: Take off your helmet, baka!
If not: eat/drink/smoke as usual.
It's one part function one part good RP. A lot of helmet wearers already do this just to be RPing like pros, but I'm a big fan of gear forcing you to do it rather than allowing people to casually pound lagers and smokes through their helmet.