Many devout brick and mortar players say you can't "read" online players. I feel there are many traits, especially if you make use of player notes that most sites offer. Some that I have found and made note of on a regular basis...
1 If a player, especially in a case where they have been betting a hand since before the flop, take an inordinate amount of time on the river, HE HAS THE NUTS AND IS DOING DRAMA, to drag in the not-so-attentive players.
2 Watch the pace of players and make note. Some are slow, some are fast. When you play with the same players, you can tell when "they have the nuts" by a distinct change in the pattern of betting.
3 Players that buy in with abnormally low buy ins will often try ramming and jamming a pot. I know many that will buy in as little as $5 in a 1/2 game, willing to lose it three or four times, figuring one good pot more than makes up for it.
4 Watch for players that always come into the pot raising and suddenly just call preflop. It is a good signal they have a weak piece of trash....or weaker than their normal piece of trash
5 A player that checks quickly, the majority of time it is signalling they have NOTHING and notice it immediately.
There are many, but you need to make notes on players. It is NOT a waste of time. I use shorthand notes....
DNB = Do not bluff....... this players will call with anything, but be sure and value bet them.
CTR = Calls to the River.... basically the same as above
NBS = No Board Sense... the player only sees what he has, bets/raises paying no attention
FISH= the obvious, bets, raises, calls, overcalls with garbage
CAPS= has to control the betting, always gets the last raise in even with weak draws
TILT= A player whose game totally changes with a single bad beat (his opinion of a bad beat)
TURTLE= He plays scared until he has the nuts, if he bets, throw it away
Short notes make it easy, I also make note of unusually high buyins or low buyins. If you know a player has been buying in big in a 2/4 game and now he is playing 1/2 or lower, he probably has little respect for raises, or money in general in a smaller game.
If a player always buys in low, more times than not, he plays timid until he has the nuts.
Hope this helps some of the newer players. Many of you long time players know a lot of this.
Good luck