It’s not good enough to be a favorite to have the best hand when determining whether to bet on the river or not, you must be a favorite when you are called. And it’s best to size your bet to an amount that gives you the best EV possible.
In some situations your opponent is inelastic. In other words it doesn’t matter how much you bet; he’s either calling or he’s not. But in most NL situations our bet-sizing generally determines our opponent’s calling range. And varying bet-sizes have varying degrees of EV.
The expectation from your value-bet is the percentage that you are called and win times your bet size, minus the percentage that you are called (and/or check-raised and fold) and lose times your bet size. For example: If you wager $100 with a 25% chance to acquire a fold from hands that you would beat if you checked, a 50% chance of winning if called, and a 25% chance of losing, your EV on the bet is +$25 ($50-$25). If in the same situation you made a larger wager of $200, but got called less frequently, your EV would be different. For example: Say your opponent now folds 40% of the hands that you would beat if you checked, and calls only 35% of the time you have the best hand. Now your EV is only +$20 ($70-$50). Your larger bet has increased your swings and lowered your expectation because you’ve reduced his calling frequency.
Of course, accurately estimating all this in the heat of battle is tough even for highly experienced players. You must estimate your opponent’s calling range and determine how your hand relates to that range. If, last to act on the river, your hand beats 55% of that calling range, you should generally bet. If it’s 45%, you should generally check.
Yes, I purposely simplified the problem. Real world poker equations are typically more complex with more factors such as check-raising, bluffing, image issues, positional considerations, etc. But you need to understand this basic concept from which to grow your thought process. Before value betting, think about how your hand plays against his calling range rather than just contemplating if you have the best hand or not.
Do it correctly, and you’ll get way more value out of your value-bets.
Play Well and Good Luck