Imagine that the evidence of someone's lunch is lodged right between her front teeth, and the struggle begins: Do you tell the person? You have to pull it off with grace so both parties can remain dignified, but that's tricky, especially if you barely know her. And by that time, would you seem like a jerk for waiting so long to say anything?
Illustration by Sam Woolley.
For the "do you tell the person" question, there's a simple rule for that. If the person can immediately do something about it, you tell them. If they cannot, you don't tell them. As this Washington Post article points out, this is informally referred to as Miss Manners' You Have Spinach On Your Teeth law. Since mischievous spinach or a chocolate chip can be immediately dealt with, you would tell the person.
Now how exactly you tell the person is just as delicate. Depending on who this person is, Maralee McKee of Manners Mentor suggests saying:
If it's a friend or family member, you can say, "I always want to know when I have something stuck in my teeth. Do you agree?" As long as the answer is yes, say, "I think you have something in yours." Notice that your saying "I always want to know when…" lets the person know it's happened to you before.
If you think the person won't take it well, HowStuffWorks warns against using "you", as it might come off more judgemental than intended.
Instead, try, "There's something there" while pointing to your teeth. With family and close friends, an agreed-upon catchphrase like "tooth check" tactfully conveys the warning.
In some situations, you might opt for nonverbal communication, like catching the person's eye and pointing to your own teeth.
On the other hand, if someone pointed out something in your teeth, it's better to excuse yourself to go to the bathroom and deal with the mess there. I definitely appreciate it when someone tells me, even if I get a bit flustered at first. The way I see it: It's better to be embarrassed momentarily, preferably with someone I know, than to unknowingly have crap in my teeth while smiling at strangers the entire time.