The friend who tells you there’s spinach in your teeth before the photo is worth ten who let you smile.

Kindness that avoids all awkwardness is not really kindness. Telling someone something small and embarrassing costs you a moment of discomfort and saves them a larger one, and most people decline that trade because silence is easier. Who is willing to pay the small cost is a decent guide to who will tell you the bigger things.

Be the one who mentions it, quickly and without ceremony, so it stays small. When someone does it for you, thank them rather than getting flustered, so they learn it is safe to do again. The whole practice depends on the reaction it gets.