Where I am from there are so few atheists I don't ever assume anyone is an atheist. If I am ever curious though, I have on a couple occasions made mention of the flying spaghetti monster. Odds are if they are atheists they'll know what your talking about, if they're christian they just won't know what the hell your talking about and ignore it. Closest thing to a secret code-word I can think of :)
I've certainly used that one, with decent success. I find the completely bafflement of the religious people rather amusing. I've also brought up the invisible pink unicorn on occasion, and when they ask what on earth that is, I just tell them to look it up on Wikipedia, and leave them to it.