Understand that poker players who are good at bluffing will make it a point to look more intimidating when they a weak hand. They will try to sound more confident, slam their chips instead of placing them gently, and stare at you, as if challenging you to call or even raise the stakes. On the other hand, keep in mind that they will do the exact opposite if they find themselves in a strong position. After getting a monster hand, they will try to look as passive and meek as possible. You will see them showing obvious signs of disappointment such as sighing audibly, shaking their head or slumping in their chair. Try not to fall for these performances.
Observe the shoulders of your opponents closely as they view their cards for the first time. Tension builds up in the shoulders when a person is under stress. If you notice a slight drop in their shoulders after they have viewed their cards, it is an indication the tension has been released. The most likely reason for this is that they have not gotten a good hand and therefore are not tense or stressed about their hand anymore. Sometimes, however, a great hand also acts as a stress-reliever, which consequently leads to a shoulder-drop; usually, however, this is not the case. Therefore, consider a sudden relaxation of their shoulders as a sign of disappointment and prepare yourself for a bluff.
Some people become stressed when they are lying, which is why their pulse rises and the breathing becomes slightly deeper. Observe your opponent for these signs when you are trying to figure out whether they are bluffing or not.
If your opponent is an introvert and normally a quiet person, but suddenly starts talking more than usual and being a bit animated, then there is a strong chance that they are feeling extremely relaxed because of a monster hand. Therefore, expect them to be bluffing if they say something like “darn” or “I hate my luck!”.