The most reliable way to determine what a player has, online or offline, is picking up on betting patterns and hand selection. Also keep an eye on position and what plays they use against certain players. Keep it all in mind while figuring out ways to exploit them.
I do not, on the other hand, agree that some players can not be read by how long it takes them to act. Most players are not going to go to the bathroom or dashing to pick up the phone during the middle of a hand anyway. Especially if there is a considerable amount of money on the line.
That being said, there are certain players who make defensive checks or even a quick check to enduce a bet. Others will do a delayed reraise in an attempt to appear weak for a call. I'm not saying act on these the first time you see a player use them - but simply keep an eye on these things and a pattern in the hands that were shown down.
That all being said, I'd like to recommend that you act at consistant intervals. Which, basically means, avoid using the auto-action buttons. Phil Hellmuth's book also recommends that you empose a "10 second rule" or something along those lines - which I don't follow.
All in all, just as in live poker, there are no hard and fast rules that can be applied to all players as far as tells go. Your ability to read plays needs to stem from observation and identifying patterns.
-money