when to call
One of the most exciting things about NL Hold 'Em is that you can go all-in or be put all-in at any time. So losing any one hand can make or break you, especially at a tournament.
So one of the most important skills is knowing when to call. Conversely, knowing how much to be on the river is key.
Sometimes it's hard to tell. I get beat so much by flushes (lost two straights just last night when a flush showed up) that I tend to run and hide as soon as a bad flush draw shows up.
However, the other day in a tournament I was in a hand where I was pushing bets and the guy kept calling. He looked like he was fishing, there are two clubs and two diamonds on the board. We each were around twice the starting stack total (3,000 to 1,500 start). When a club showed up on the river, he instantly tossed a 1,200 bet into me. I thought about it for a second, and then called. I won with two pair.
I don't know why I knew he was bluffing. It wasn't a value bet, because he knew I was deep into this pot and couldn't be scared away at a couple of black chips. I guess it realized it was more of a bet to scare me away than anything else. If I had made the flush and wanted him to call I'd probably make a somewhat smaller bet, maybe 600-900 as the pot was already around 1,500.
How do you tell, online, when someone is totally trying to bluff you off the pot?
Keep in mind, this is a rare instance. I usually end up not believing and get burned.