Reading is an art, you have to work at it, even then some guys just can't get it. Like Helmuth says "Try to guess the players hole cards and see if you were right" You would be amazed at how close you can get in live games, forget about the Donkeys online though. Some players are easy to read and others sit there like a rock giving you nothing, take advantage of the ones that you can read. Last time I played a B&M game there was one kid who would only bet with a flush draw, he would check-raise with a strong hand, I always re-raised him when he bet and made alot of my profit off him. The guy on my left was easy to read by his breathing, you could hear his heart when he had a hand which saved me alot of bets, lol. The guy on my right would start shaking his leg when he was on a draw and betting but not when he had a made hand, unfortunatly he drew out on me a few times and left the table with my money leaving his seat to an old guy that has been playing poker since before I was born and I couldn't get a read on him. I am in sales so it is my job to read people so I feel that I have an advantage compared to most people but if you pay attention you will see some that will make you profit or just as important save you money. I have one buddy that has a read me but he won't tell me how, imagine that.
Online is all about betting patterns. Pokerstars
has instant hand history which will tell you alot if you take the time. Take notes of your opponents after reading the hand history so you have info for the next time you meet them. You would be suprised how many times you will run into the same players even though there is 100,000 online at a time. If a player is really bad (or drew out against you all night) then add him to your buddy list and look for him the next time that you sit down.