While the tips given here may help you discern some lies from truth, it is in no way a failsafe method to detect deception in others. In fact, studies show that most people can spot a liar 54% of the time. You might as well flip a coin.
Possibly the largest problems with the polygraph, and why it has been inadmissible in most U.S. courts, is the notion that jurors give the test too much weight, particularly in light of its noted inaccuracies.
So why do we lie? Sometimes, it's done to embellish the story about the fish that got away. Sometimes it's intended to shelter someone who would be hurt by the truth (i.e., "Yes, Virginia, there is a Santa Claus.")
Maybe you are confident that you are a terrific lie detector -- you are sure you can separate the trustworthy from the slime. Surprise! More than a dozen studies have shown that there is no link between confidence and skill.