There was an interesting comment made in another thread about online poker -- comparing it to live poker in casinos.
True. With online poker gone are the "tells" that Mike Caro made his study of, and wrote his book about. But there are other "tells" in online poker including bets, chatter (comments), and knowing the history of how players bet.
With online poker you can't watch a player's eyes or see how he is handling his chips or whether he looks at his cards more than once. But playing the game online and remembering the patterns of players can give you plenty of tells or signs and signals of what a player has or is holding.
At various sites where I play free games (I won't pay for online games because I don't trust the technology of the sites -- they could knock you off the site mid-hand or during the game) I know how many players "play." I know which players tend to raise when they have picture cards, and I know which players tend to go all-in pre-flop based on what pocket cards they might have been dealt. I know which players play small cards and which play only "big cards."
There are different tells in online play.