Are you straightforward in asking for it, or do you try the "Oops, I don't know that got there" bit
Straightforward; you'd have to have a pretty small dick or the girl would have to have a large, gaping asshole for a sneak attack to work. And if she does possess such a cavernous maw, she's probably willing to take it in the shit vent. Again.
Why a sneak attack? Just boldly put your dick in her ass. If she asks why, tell her that you are gauging her "girlfriend potential" by her reaction. If it is anything other than enjoyment, just shake your head and mutter to yourself when you take it out.