I just come out and say it, polite and usually in some joking way. But I almost always say , hey I would want someone to tell me or let me know..
Word of advice, no matter how good my intentions are, it doesn't go over well with the girlfriend. The fear of her stabbing me in the face and saying "how about now" or I'll here about it for 9 days, or all the grief and money it will cost me, to help whatever it is, sometimes it's not worth it. Lol
I would want to know if I had anything like my fly open, so I tell people, in a cool way.
G