How does Bond know that Le Chiffre is bluffing?

As for how bond finds out Le Chiffre is bluffing was due to the aspect of the previous game they play (one before the game where Bond actually loses everything) where Le Chiffre simply ups the ante with nothing but wins the game at the dealing of the river. At the top of my hat I believe this is the dialogue which ensues between Bond, Vesper and their connection after that.

Bond: Le chiffre is bluffing.

Vesper: But he had the best hand.

Bond: Which he got off the last card.

EDIT: I reread the question again and I completely overlooked the aspect that you didn't have much of poker knowledge I have updated the information to include some poker tips together with the actual process which lead to Bond figuring out that Le chiffre was bluffing.

If you look at the card sequences mentioned here for the first game, Bond mucks off (he throws his card away not wiling to challenge Le Chiffre's. At that point we also don't have a glimpse of what Bond had as well) the river but if you see when the first 3 cards were dealt, Le Chiffre had nothing to raise the ante even if you include the 4th card it was just 2 pairs which was fairly quite a weak hand.

The point I am trying to drive at here is that Le Chiffre pretty much made a game out of it with nothing on his cards, which made Bond surmise that Le Chiffre is in desperation to get the money and he is resorting to bluffing.

Le Chiffre being a poker prodigy himself finds out that Bond has read his move and uses that move to his ploy in the next play and totally deceived him. I would say he totally beat Bond in that game because he had his fourth jack at the fourth card dealt and Bond only had his full house at the last hand. Le chiffre already knows he has a much better chance of winning before he even saw the fifth card and acts as if he is bluffing to get Bond to raise more money to call his bluff. Four of a kind is a second most rare hand next to a straight flush which Bond gets in the last game. After Le Chiffre wins he even remarks to Bond if he had thought that he was bluffing slyly after that game.

As for the last game. I think Bond clearly had an edge on that one with him getting a straight flush right at the fourth card whereas Le Chiffre had his full house only at the last card dealt. Le Chiffre gets owned because of the amazing sequence that is the straight flush but I don't think he even thought about bluffing at that point because of his confidence of beating bond earlier. This scenario is a complete reverse of the previous game where Bond pretty much has the best hand possible at that point and suckered Le Chiffre to bid higher who also coupled with a really good hand he had at that point. The last poker game was a fantastic sequence for poker lovers but I think it's quite incredulous to get in a real poker play.

PS: Although I do recommend you to watch the sequences once you learn how to play poker. I think you will enjoy them more than you think.

Since you are not aware of the nuances of the game Poker, I won't delve into it and restrict myself strictly to what was shown in the movie.

If you have observed, Le Chiffre used to place his left hand near his wounded eye at times during play. That was his physical tell.

A tell in poker is a change in a player's behavior or demeanor that is claimed by some to give clues to that player's assessment of their hand.

Bond knew he could beat Le Chiffre as he was confident that he would catch his tell and get an insight into his game and strategy (bluffs included). This is what happens at the end!