HOW DO YOU TELL..?

I have been wondering: 

If a woman(not your girl friend or wife), mistakenly sits on a dusty surface and gets up afterwards, how should a man tell her that her bum is stained or rather, is it proper for a man to tell her that her bum-bum is stained?

Okay, see this:

Is it right for a lady to tell a man that his trouser zip is open when the man does not know or how should a woman tell a man that his trouser zip is open?