As in they have spinach in their teeth, they spilled on themselves, they need a tissue, etc.? Or is it best to not say anything at all?
This is such an awkward situation, but it really doesn’t have to be if you “help” someone with sincerity. If you are with friends, or your significant other, then it’s easy. Do not make a big announcement, or make a scene about it, simply lean over and discreetly and quietly say, “Excuse me, but you have a bit of spinach in your teeth.” They will be grateful, excuse themselves, and fix the problem.
Please do not put your hand up and whisper behind it to the person. It’s very rude in a group setting to whisper behind hands. It signals you are superior, and that others are not in on whatever you’re talking about — even spinach in your teeth. If you’re on a first date, or at lunch with your boss, then use your discretion. How bad is it? If it’s pretty bad, then politely say something like, “Excuse me, but you have something in your teeth; it happens to me all the time.”
During these crazy days when we are all working remotely, and we see each other most on our screens, etiquette stays the same. For example, if you’re in a meeting online, and you notice one of your co-workers has something spilled down her shirt, text her. Do not mention it to her in front of the group. Text her, and she can change her position to minimize the view of the spill. So many of us are taking care of children and pets and trying to work, all from home, at the same time. Things happen. Be kind, and show a little empathy, in person or online.