How good is this guy?

[Bad Lobster]

When you play a game against a table full of strangers, how long does it take you to pick out the good players? And what do you look for, short of observing someone for hours and hours?

In a low-stakes game, where you don't assume the opposition are all experts, a few players will quickly identify themselves as bad by making terrible plays. You can rate few others as at least fair if they go a while without doing anything obviously dumb. But that doesn't mean they're experts.

Can anyone think of a single play or series of events that would make you watch out for a guy right from the start?