This is why many pros wear sunglasses or visors/caps when playing, they know that the eyes rarely lie. For eg. many players can't help but stare at big hole cards, so their length of time peeking is longer. Conversely, if a player is looking to steal the pot, he may look to his left to see if the remaining players, who haven't yet acted, have quickly glanced at their cards and are likely to fold. Another eg., a player may try to ask you questions about your hand, knowing that people rarely can "look someone straight in the eyes" while being dishonest.