Such a wide question..
In my humble opinion I'd say you would need to keep in mind at least 3 important things, either for the flop, turn or river:
- Your position
- How many opponents are you facing
- The type of game you have been playing on that table
Apart from that, could be a lot of implicit variables, so like I said, I don't know if there actually is a correct general answer to that wide question, more than focusing on certain points.
If it is sometimes difficult to make decisions on the river and even more so when the other player goes all in and makes us wonder if he is bluffing or if he really will have a good hand. That will always depend on what type of player you face and the board of the table, you must analyze the hand as it developed from the beginning and the possible projects of the villain's hands to try to have a better reading.