Calling on the River
Making close calls on the River is an art that takes a lot of consideration. The following factors will influence your decision:
The relative strength of your hand. If you hold As4d and the board shows KdQdTd8d3h, you would have a sure fold. All you have is ace high, with a strong possibility of a flush, a straight or a high pair. Even with a two pair or a set you might consider folding your hand. If you had a weak flush like 5d, you might want to call.
The number of your opponents. If there is more than one player left in the hand, you should be more inclined to fold a weak hand.
The probability of a bluff. How often have you seen your opponent to be bluffing? How often would he bet a very coordinated board?
The texture of the board. If the flop had only one flush card, but turn and river have made a flush possible, then the danger of a flush is lower, and you would rather call. If you see a flop with 3 flush cards, the probability of a flush must be considered.
Your opponents reaction. If an opponent who previously just called and suddenly becomes very active, bets and raises, then that is a strong indicator for a completed draw and you should think about folding.
The size of the pot. If you are looking at a big pot on the River, you should almost always be prepared to invest one more bet to call.
The size of the bet. In a no limit hold'em game, you would certainly tend to call a single bet. On the other hand, many players make small bets with strong hands in order to keep their opponents calling. Some players know that and might try to make small bets look as if they were bluffing. In a fixed limit game you should call more often, based on the relation between the size of the bet that you have to call, and the size of the pot.
The number of players to act behind you. If you are last to act, it is easier for you to call. But if there are several players to act behind you, you should rather fold - although sometimes you should even consider raising in order to eliminate a hand that may be weak, but worse than your own.
Your position in a tournament. If you play a tournament, your decision will very much depend on the size of the bet relative to your chip stack.
Consider all those factors, and in a situation where you are confronted with a relatively small river bet and a rather big pot, remember that in this situation, you can make two mistakes: the first mistake of making a bad call can cost you one bet, but the second mistake of making a bad fold can cost you the whole pot if you are ahead.
Both bad players and strong players are perfectly able to bet a busted draw. In addition to that, a weak river call will be noticed by your opponents, which may lead to reduce their tendency to try and bluff you on the River.