Tips for reading players online
Watch everyone at all times.
As Otto von Bismarck said, "Only a fool learns from his own mistakes. The wise man learns from the mistakes of others." The absolute best way to read people is by watching what they do with other people in the hand. You don't want to waste a bunch of chips trying to push someone out of the hand when they've got you beat the whole way thanks to a brilliant trap.
Disclaimer
I am not good enough to track all this at once. These are just tips I’ve found, each on an individual basis.
Reading All-in players
Don't fall into the trap of assuming someone goes all-in with any two cards just because they go all-in two hands in a row unless you end up seeing their hole cards. Some people like to go all-in, but anyone can catch a couple monster hands in a row. If you do see someone who goes all-in and has to show and they have bad cards, don't assume they always have bad cards when they go all-in. Could have been a failed
bluff, they could have been on tilt, or next time they just may get lucky against you.
Calling Stations
Pay close attention when someone calls all the way down to the river. Do they win the hand because they were trapping? Do they lose to top pair and have to muck? If it's the second then you may have a calling station on your hands. When they're in a hand with you, don't try
bluffing, they'll just call it all the way down. When you have a good hand against them, milk them for as many chips as you can get out of them. Usually by starting with half the pot and slowly raising, instead of constantly raising pot-sized bets.
People who love draws
Look for people who call or raise to the river and then fold on the river. There's a good chance they had a raw and missed it. In that case, label them as someone who loves to draw out. If they stick around in a hand, you know there's a good chance they have a draw. Wait to see if the draw misses and then push them out of the hand on the river. But be careful, because even if they are chasing a draw they could still have a decent hand (top pair or so).
Bluffers
You can only really identify a bluffer by witnessing a showdown with them. This is why it is very important to watch how they play against other players. If someone shows you a bluff intentionally, they are trying to trick you into thinking they bluff a lot. Don't fall for it. If someone shows a bluff at showdown, or if someone bets decent and then folds to a re-raise, then you can mark them as a bluffer. But again, even a bluffer can catch a legit hand against you.
Rocks (Solid players)
When someone plays few hands, folds often when they miss the flop, and win most showdowns they get to, then treat this person like a "Rock" (a real solid player). The way to take advantage of one of these players is to c-bet and hope they missed the flop. If they are willing to call on the flop, don't bother betting into them unless you've got a great hand because they are only calling with good to great holdings themselves.
Shortstacks
Ignore how people play when they are a shortstack (10BB or less). People alter their play drastically and will tend to shove more often when they are a shortstack, so if you make notes, be sure to annotate it with ss to remind yourself that was how they played shortstacked and it does not necessarily apply to their normal or large stack play.
Betting Delays
When someone takes a long time to think about a bet or re-raise, it may be one of two things. They either betting a great hand and are trying to sucker someone into calling, or they have a crap hand and are trying to bluff. Yes, it can be either. So watch the hands they delay on and when the hand is over, then decide which they were trying to do and make notes on that for later. Also, make notes on what they were doing when they weren't bluffing.
Trappers vs. Bluffers
When someone calls to the river then bets big, there’s a good chance they are a trapper. Pay close attention to whether or not they actually have a good hand when they do this. If so, bet against them with care. If not, you may be able to call safely when they behave like this. Maybe.
Conclusion
As with everything online, it is impossible to predict how things will happen the way pros can do it face-to-face. But perhaps some of this might give you a slight advantage in a key spot.