It's awkward no matter how you say it, and the most important thing in this situation is to communicate clearly. You want the person to hear and understand correctly the first time. Don't take a chance on a joking colloquialism.

Euphemisms for "your fly is open" tend to be joking. The particular joke or euphemism may even be a family thing. I'll contribute this one: in high school, a friend of mine had a teacher say to him "You'll catch pneumonia!" What is that? Was it the teacher's private joke? Maybe the teacher's dad said that to him once. For obvious reasons, it's not something you say or hear very often.

The person you're talking to may not understand "XYZ" if that's not the particular joke he heard. 

I really believe that you should say something plain and straightforward, like "your fly is open." If that seems too direct, "check your zipper" (the whole phrase).

Anything else is likely to get you into a long conversation, "XYZ." "What?" "XYZ." "XYZ?" "Examine your zipper." "Examine my... why should I... OH SHEESH, OMIGOD, SORRY SORRY SORRY SORRY..." It will be twice as embarrassing to everyone as just saying it plainly.

You forgot to zip up/ Your fly's unzipped