Or, There Is Something In the Way OF Your Smile.
When faced with particles in your companion's teeth, let them quietly know without interrupting conversation or drawing attention to the situation. You can say whatever you like as long it is direct, and it is best to avoid wildly gesturing or miming. "There is something in your smile," or "You have something in your teeth," work very well.
If the tables are turned, first take a sip of water. Hold your napkin up to your face while you try sucking the bugger out, and depending on your relationship to your companion you can ask them if its gone. Otherwise go to the restroom and take care of the situation with the help of a mirror.
It is always best to let a person know and save them the embarrassment. But then again, it happens to everyone.