“Should I tell my boss when his fly is undone?”
Here is the short answer … No. No more than you would tell anyone they had parsley in their teeth, nor say “bless you” (or any variation thereof) after a sinus eruption, even if it makes it onto the Richter scale.
Does this make you uncomfortable? Consider this: it’s all about saving face. And while you might be chomping at the bit trying to point out these … shortcomings … you must be mindful not to embarrass the other person by letting on that you have noticed.
Now, I am not a fool. (It’s worth mentioning.) There are circumstances in which there is no recourse but to help someone out: a spouse for instance. Consequently, I recommend that you adopt a system of discrete signals with close friends or family members to draw attention to a bit of food-on-face should the situation arise.
For instance, in our family, we use a code word which means absolutely nothing but that can easily be slipped into a sentence. People “in the know” will hear it and remove the offending crumb/drip/spot whilst people who don’t know will simply think they have misheard.
Sadly, I cannot share with you exactly what it is because every member of my (large) extended family now knows it. Needless to say, it’s getting awkward at gatherings when the whole family simultaneously reaches for napkins at the sound of the word. It sort of freaks the other people out, in fact. Pavlov would be amused, though.
So I encourage you to get your own word, and share it with those you love.
Depending on the situation and if dealing with someone particularly switched on, you might be able to manage a “Sir, a word please?” followed by “I thought you might like to borrow my mirror for a moment.”
But, when it comes to your client, a new acquaintance or employer, unless you have a very privileged relationship, then I’m afraid that you’ll have to just put up with the problem. And try not to stare.