An immediate, significant bet will sometimes be a bluff. One spot you will often see this is when a player has bluffed or semi-bluffed previous streets and has “painted himself into a corner” on the river so that he feels “forced” to bluff.
For example, a pre-flop raiser might continuation-bet the flop quickly, semi-bluff the turn quickly, and fire out an immediate river bluff. This is a common pattern for bad players who want to appear confident and who aren’t conscious of how their actions appear to others. If you notice this pattern it might clue you into a bluff.
This leads us to another factor in bet-timing tells; how hard the board is to read. In most significant spots, an immediate bet is unusual whether a player is first to act or not. But an immediate bet is even stranger if it comes soon after new board cards have arrived. Most players need at least a few seconds to absorb how the new card or cards have changed the game.
For example, let’s say you’re heads-up in a big pot going into the river, and your opponent is first to act. The river comes and your opponent bets out immediately, without even seeming to consider the board. That is definitely much more “strange” a behavior than if you had been first to act and he had bet immediately after you had checked, because he would have had more time to think about his decision.
Some boards are really easy to read, even when a new card arrives. For example, a player who’s been betting his flush draw the whole way and then makes it on the river doesn’t need to think about his bet. He’s made his hand and he is capable of betting immediately. Same with a player who’s flopped a set; there’s usually not too many turns or rivers that will make him second-guess a bet, so he is also capable of betting immediately.
Other situations require at least a few seconds thought from most players. If a player is betting an overpair the whole way and a four-straight comes on the river, he’ll probably have to think for a few seconds about his action. Or if he backdoors a medium-strength flush, he’ll probably have to think for a little bit.
The complexity of the board is another factor you can use in trying to distinguish a bluff from a value- bet. Mostly, I would use it in context with the pattern mentioned earlier, where a player has bet multiple streets and is in a spot where he could feel compelled to bet the river.
For example, let’s say you’re in a pot with a weak opponent who has raised pre-flop and bet the flop and turn. If a four-straight comes on the river and he fires out a large bet immediately without even seeming to think about how this card might have hit your hand, this is one clue that he is bluffing.
To sum up, most of the time, significant immediate bets will indicate strength. Sometimes it will be a bluff, but usually only when a weak player has “painted himself into a corner.”