Yep. The best clue, aside from body language, is that they are paying attention to each person in view, no matter how far away or nature of activity. The really good ones actually try to act like they aren't looking around, but those are also often the ones who aren't terribly good at looking like they are shopping. I'm sure I sometimes think people are undercover agents who aren't, but I doubt there have been many that slipped past me who actually were.

