When you play a live poker it is easy to read the poker tells of the opponents as you are face to face. You can easily know his emotions and play the game and win game. But this is not the case with online poker. Here the opponent will not be face to face. You wont be able to read his emotions that easily.
There are techniques to read the opponents emotions while playing online also. The major tell is the betting strategy of the opponent. This tells the emotions and cards of the opponent. If a player is raising his bet in every hand and on the river if his bet is less then it says that the player hand is not strong. He is acting as if he is strong. The players betting speed also conveys the emotions of the players. This tell cannot always be perfect. A player who is strong sometimes tries to show his strength making the opponent think he is not strong. Sometimes the player with strong hand acts as if he is not strong. This cannot be identified. It requires experience to understand the situation and various tells.