How can you tell if someone is a binge drinker?

If they...: ...Have trouble stopping once they start drinking, that's a pretty good clue. Doing something while drunk that you either regret or forgot is also indicative.

Seeing them: Binge drink, or if they tell you they are binge drinkers. There are no physical clues to someone who binge drinks, unless you see them when they are actually doing it, or recovering from it. Typically, the binges get longer and longer, with less time between, and eventually they are drinking all the time. Some stick to binges though, and can hide it for a very long time (sometimes forever).