Poker is not a fast game, nor is it an exact science, a good player will turn on the aggression from time to time depending on many different factors, like his Chip stack or Position on the table, Who he feels are tight players etc, and at times he will slow down his aggression to keep players guessing on his playing style.
When you constantly come up against the same players, as in a Cardschat Free-roll, you can on some site's make notes on players,
pokerstars allows you to make notes, and you can change the players Avatar border to a different colour, for example Red for Aggressive players, or Blue for Passive/Tight players and of course you can change their colour if they start to show an alternative style.
A good player will always change their style, when playing, to keep you guessing, but then again so will bad players, depending on their hole cards, so it is never going to be easy.
Getting a read on a player when playing on-line is extremely hard as the only real information you have is their betting styles, you cannot see the little eye twitches or throbbing veins in their necks, as you can in a live game, so it all boils down to intuition, and you have to go with your gut instincts, I have made some very dubious calls and won hands even with just bottom pair or an Ace High hand. That takes a long time to be able to make those calls and many thousand's of hand's experience, Some times I am right, some times I lose, but usually when I am wrong my Tournament life is not in jeopardy, and I can still make a come back, even with a short stack.
That is why I love playing Poker! It's Exciting!
When the fun Stops, STOP!