Look for people calling down light. Watch the hands that you aren't in and see who seems to call,call,call with something like Top pair, crap kicker.
I don't have a HUD, so I can't really tell you in terms of the stat numbers. In general, I would guess a low AF might be as if not more important than high VPIP, though I could be wrong. They work together for your advantage: the high VPIP means they will have sub-optimal cards which will tend to catch mediocre hands, and the low AF means that they will call often with these mediocre hands rather than betting or folding them. You want them calling with their mediocre hands.
Then there are the aggro-fish who will bet with anything. They tend to be obvious after an orbit or two (or less) because they always seem to be involved in half the hands with half their stack.