You're meeting friends, and when everyone shows up – all smiles – you notice one of them has something green stuck between two teeth. What is it? Spinach? Arugula? Seaweed? That doesn't really matter; the bigger question is, should you mention it?
Here's what drives our answer: When that poor soul has a look in the mirror (or feels something awry) hours later, he or she will probably wail, "Why didn't someone tell me?"
In the grand scheme of social faux pas, being unaware of something stuck in your teeth is at the very low end of the scale. At some point it happens to everyone, and it harms no one ... but it's very, very embarrassing when it happens to you. Although it's normally very poor etiquette to offer unsolicited criticism of someone's appearance, something stuck in a person's teeth is virtually always unintentional and easily fixed. Most people would rather know than walk around all day flashing a green grin.
How to pass along the message depends on the circumstances. No matter the setting, be subtle and private. The whole point of saying something is so other folks don't notice. If you're nearby, catch the person's eye and surreptitiously point to your teeth. That usually gets the message across. If a game of charades isn't feasible, try to get the person alone for a moment. If you can swing it, a quick trip to the restroom is ideal because there'll be a mirror to help with the extraction. If not, the miracle of smart phones with front-facing cameras may save the day.
However, if you're trapped in a large group, it makes more sense to quietly offer your information while just walking by. Keep your language simple, direct and nonjudgmental; avoid using "you" because that can come across as a reprimand. You might say, "You have food stuck in your teeth," but the offender hears, "You have terrible oral hygiene." 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.
Not brave enough to say anything? Casually ask, "Hey, do I have something in my teeth?" Often the person will ask you to reciprocate. Now you've been formally asked to make the observation – problem solved!
If you're really just too flustered to say it, send a quick text or private message.
Keep your advice short and sweet, and anyone – friends, acquaintances, coworkers or even strangers – will appreciate the intervention. In the case of strangers, don't invade their personal space, and don't stick around to see what happens next. You've done your good deed. If you're having a conversation with a friend, make your statement and then go back to your chat. Less time on spent on the topic equals less embarrassment for all.