Relative to live poker, there are far fewer tells that you can pick up on to find out whether or not your opponent is bluffing, especially in online Texas Holdem. This is for the obvious reason that you are unable to physically see who you are playing against.Tthere are still a number of tells that you can look out for online that can give away the strength of your opponent’s hand. 
If you Long Wait and Check it’s mean this shows that your opponent is weak, because he wants to make you think that he is considering to make a move with his hand when really he wont be holding much at all.
If you Long Wait and Bet/Raise it’s mean this is the opposite of what was mentioned above. This action shows strength, as your opponent wants to make you think they are deliberating a check because they have a weak hand, but in actual fact they are holding a strong hand.
If you Check/Call, Bet it’s mean you are in position, your opponent checks the flop, you bet, opponent calls your bet, and then your opponent bets out a relatively small amount when the turn comes. In this situation you can comfortably put your opponent on a certain basket of hands.
If you Instant Bet/Check it’s means almost every site will have action tabs that you can select before the action gets to you. This means that you will act instantaneously when it is your turn.
These are the most useful tells that I have been able to pick up on in my time of playing online poker.
The more you play the more you will understand the logistics of the game and be able to read people by their betting patterns.