I've had to say something to guys I didn't really know before, and usually it's no big deal. If it's a younger guy, I can say it in a fatherly tone and it's not creepy. If it's an older guy, I'm more like, "Excuse me sir, but I don't think you realize..." Very respectful. The ones I have had to think twice about are men my age. I don't want to come across like I was staring at his crotch (although sometimes I was), but I feel like I'd want someone to tell me, so if I have the opportunity to tell him, I do.
I once was in front of a class teaching when I realized my zipper had broken. I had to stand behind the lectern the entire class, which was very limiting for me. A couple of weeks later, the same thing happened with a different pair of pants. Anyway, I went into my department office to tell the secretary I was going home to change pants because my zipper had broken. I had my shirt untucked and covering it, of course. So she says to me, "Hmm...twice in a couple weeks' time, huh? You're just packing so much in there that the zippers keep breaking." Then she laughed.
I turned red, but took it as a compliment. Fact was, my pants were worn out and I needed new ones, but if women want to believe I'm busting zippers because my package is so big that my pants cannot contain it, I'm not going to tell them otherwise. LOL