I know there ara some tips to detect tells while playing live, but how can you do it while playing poker online? What should I look for?
In online poker the only tells you can get are:
You should treat bet sizing as a FACTOR of the hand, and consider it and keep it in mind in EVERY hand, not handle it like a tell. Tells can help you win the hand, make better decisions, and are necessary to notice. Sometimes they can get you huge advantages, but USUALLY you can make optimal decisions without them.
So, when someone make quick calls, usually it's a tell (it's not a tell though, if someone always makes quick bets); on draw heavy boards, a quick call is a draw most of the time. When someone waits an extremely long time before they raise, usually they have a big, nut-type hand.
When someone makes a bet and starts talking, they are trying to make you either call or fold, but it's a general good rule that players who are bluffing try to avoid paying the other players attention, so it (might be) more likely that he wants you to call. (Again, if he talks a lot anyway, this could have no meaning at all.)
It can be a tell also if someone never makes quick bets and now he made one; this could mean he already planned the hand, before the card came, or he had such a hand he doesn't need to think at all (maybe the best or worst hands can be these types of hands in situations).
These are the types of tells you look for in online poker. But be aware! A quick change in the players behavior doesn't always mean something, if you don't know what the opposite thing means with this player. And it's obvious but when someone says "I have to pee" obviously it has nothing to do with the hand (except that you can steal his big blind on the other table :D)
Oh and I almost forgot, you shouldn't always rely on timing tells because connection issues can cause them too...
Firstly, I'm using my words very precisely here as this area is rife with poor distinctions and that itself is due to the subject material.
"Tells" are what I consider traits that are outside of what you should consider "Game Mechanics" or (GM). GM's are all the explicit things you'd see that are non-player specific and intrinsic to the workings of the game, i.e. bet-sizing, bet-type, position, stack sizes, game-type, etc. The distinctions are subtle in many cases. Certainly, some of these can be considered when discerning player tendencies but they are distinct due to a critical factor.
Tells are relative and implicit. Game mechanics are objective and explicit.
People like to play the amateur behavioural psychologist because it's fun. They see an article about a micro-expression, and now everytime they see someone twitch they think it's a bluff. That twitch is only relative to a baseline of actual information.
I doubt anyone could tell you that they can provide solid evidence about the timing of someones bets online. They might have nipped to the toilet for all you know! At BEST, they're unreliable.
Without being dismissive of your question, because it's absolutely valid (upvote), you'd be better off spending your time learning the truly tangible elements of poker. Game theory, and the hard landscape concerning the fundamental maths will serve you better than spending time on the ethereal vagaries of "soul reading".
For the most part, you shouldn't rely on online tells, because they can be fairly easily faked.
That said, there is one tell that translates to online play: bet size. Obviously it will vary by player, but if you pay attention you may find that min-bets and pushes say something about the size of the player's hand.
When playing fast games online (~ 30 seconds of decision time), the time a person takes to decide can be a tell-tale sign. Of course, general lag, and intentionally long pauses make this an inexact science, but on average, if a person takes too long to make a decision (longer than 2/3-rds of the clock), chances are they are unsure of their choice.
As mentioned by William, the other tell-take sign is the size of the bet, and that is somewhat driven by the game interface (i.e. how easy is it to bet double, for example), etc.
The best thing i'd tell you is to install some software that can give you information about the player. Like "Poker tracker" or "Holdem Manager" it has a poker hud which can give you statistics of what that player played over the hands you saw. This way you can know how that player plays and predict a situation.
Lets assume a player 3 bets you and you check your tracker and see over the last 100 hands you have form him he only 3 bet 5 times out of 100 hands, this way you obviously can put him on QQ ,KK or AA pretty easly
I'm not saying this is a tell im just trying to say that this is almost better than finding one.
Cuz someone who bluffs when taking 20 sec could be considered a tell at some point and in real situation (he only went to kitchen to take a beer thats why those 20 sec) and than you loose money because of that.
From a point of view statistics reflect the players style and are only abused at higher lvls like 5/10$ limits or higher.