How do you tell your girlfriend she has a fart fetish?

I'm betting she already knows, but you could say, "You know, sometimes it seems to me that you act like you have a fetish for passing gas." This will give her a chance to say, "Well, you're wrong" (if she does, just drop the subject), or maybe "Yeah, I do. Is that okay?" If you're not ready to say "It's okay" (and mean it!) then don't ask!