I'm not so sure this type of social dilemma is cut and dried, but may be more of a gray area. There are multiple types of bodily or wardrobe malfunctions that we can either point out to someone, or have pointed out to ourselves. And almost always, these are to save either us or others embarrassment.
You can name them as well as I can. Pants unzipped, dress stuffed into panty hose, wet spot on crotch, toilet paper stuck to your shoe ... the list is almost endless.
The gray area comes in with who, when and where. Who do you tell, when do you tell them, and is it the appropriate place to tell them? Hopefully, significant others always have one an other's backs. Jan will tell me if I have something hanging out of my nose, which I don't think is that often. And I'll tell Jan if she has dried toothpaste around her mouth, which is almost an everyday occurrence. She gets annoyed at me for mentioning it, but I think she is secretly grateful, because I'm sure she doesn't want to go out in public looking like she just consumed a powdered donut in one bite.
But what do you tell someone you don't know? I'm not necessarily going to tell the cashier at the grocery store that they have a booger hanging out of their nose. They might knock it off onto my food. And I'm not going to tell someone walking down the street that they have a wet spot on the front of their khaki pants. I might get punched in the mouth. What about someone at work? Maybe that depends on if you like them or not.
A few years ago, I worked in an office that employed a number of young women in their sales department. One day I was walking by one of them and noticed that she had a glaring red spot at the "junction" on her white slacks. Okay, now what do I do? Obviously she wasn't aware of it, because she was walking around talking to people, and either they hadn't seen it or were too embarrassed to say anything. So, do I walk up to her and casually say "Hey Mary, I see your period has started"? Probably not. I kind of knew the receptionist, so I went over to her and told her that Mary had a problem. She took one look at her and said "I'll handle it". Later, I saw Mary walking around with a sweater tied around her waist. I just hoped the receptionist hadn't told her that I was the one who had noticed it.
And I hope I never have to do that again. Did you ever notice that when you hear or read something along this line, that you have an uncontrollable urge to go check yourself out? I think I'll go look in the mirror for nose dirt.