How to tell when some one lies to you?

Their eyes blink
They sweat
They stutter
They has many uncontrollable motions like shaky hands etc
They can't look straight to your eyes, they are normally looking down and to their left.

These are all good things to look for and usually work on the general person. However, a really talented used car salesman can look you straight in the eye and tell you a great big lie and he won't break a sweat, his hands won't shake, he won't stutter, he won't blink abnormally or have any other uncontrolled motions and people buy the car because they thought that he was so honest. It's his job to lie everyday.

There are a lot of people whose livelihoods depend on them being able to effectively tell a good lie without being noticed. Even polygraph (lie detector) tests can and have been beaten hence why they are not admissible in a court of law. Because compulsive, pathological liars and psychopaths are often so convincing that they can consistently beat the polygraph tests. And some can even convince themselves that their lies are the real truth.
These kind of people don't exhibit the physical signs of lying that the machine is calibrated to look for, such as respiration, heart rate, perspiration, blood pressure and muscle tensions etc.