Look at how often they enter the pot and from what position. Good players will normally only play strong
hands from early position. If a tight player enters the pot from EP it's an easy fold unless you've got a real monster, at least AQ or better. If somone is playing every hand from any position, you can loosen up your own starting requirements.
Also look at raises. Most loose players are afraid to raise. A player might limp into every pot and out of the blue he raises, he's probably got a hand. This can be just as telling as a tight player who sits quite for 30 hands and suddenly calls your raise.
You should pay special attention to the habits of the 2 or 3 players to your immediate left. These are the ones you must deal with when your in stealing position, the Cut-off and the Button. Are the passive? Do they defend their blinds?
I play at FTP mostly. What I like is the ability to color code my opponents. Green is the default color = no info. I tag an opponent with yellow for high implied
odds, likely to stack or chase and Blue, tight or good. I'll just stay clear of these ones unless I have a hand. I'm looking for opportunities against the yellow.