...if their fly was undone or they had food in their teeth? Personally I have no qualms about pointing this out to culprits and the response is usually one more of gratitude than embarrassment. Reason being, I have just been walking around for 15 mins or so with my fly zip undone and no-one let me know I was on display. Now, either they were admiring the view (unlikely) or they were too embarrased to tell me. OK, they may not have noticed but that's irrelevant for this query! So, would you tell them or keep quiet?