Poker in General

Generally, bets are based on the size of the pot and/or how much money the player has left. I've gotten pretty good at that kind of spot analysis myself and when I watch on television, I can pretty much tell what they're going to bet before they do it. Since I can see all the cards in play, I have an instinctive idea of how I would play it, based on what I would want my opponent to do (call, raise, fold, etc.). Do some reading on betting strategy and pot odds and you'll find yourself calling out the bets in no time. I could go into more detail, but there are plenty of more qualified people who have already written this down.