Since I don't take notes online, I look for tells.

A few things I've noticed about the majority of online players:

Speed of decisions: This can be a good “tell.” Often, a quick bet is a sign of weakness, and a delayed action is a sign of strength, as the player is calculating his strategy with what he reckons to be a serious hand.

Immediate action (boxes pre-checked):

When a player has checked “raise any” it should be obvious that he has a strong hand. If a player uses the check box to “check” then you can probably surmise that his hand is weak.

This is paticualy handy when you are in the small blind and the big blind has pre-checked the check tab, you can generally tell that he is very weak