How can you tell if someone is lying

People tend to avoid eye contact when lying.

There is a bunch load of give ways and an entire science devoted to study them...I can recall a few from memory:

- Looking up left when trying to recall facts instead of looking up to the right.
(it implies the use of imagination instead of memory)

- The use of third person speech instead of first person speech to distance oneself from the situation.

- Defensive body language.

- Excess of assertive strong speech denying the facts instead of trying to cooperate or express natural shock.

- Detailed recalling of the misconstruction's of the facts, with no lapses in memory. Normally people have tids n bits not detailed memory unless they are faking while trying to control the whole process of lying...

- Authoritative gesturing in order to be convincing. Etc etc...

The major point to retain here is that one of these signs in isolation means nothing, its the confluence of several of them together that gives away a liar.

look at the eye pupil when it gets bigger he is lying

...it may be love at first sight...or dope.

Look for their tell.

For some, seeing their lips move is all the tell you need.

Unless all you are going on is a one line statement, liars reveal lies by contradicting themselves.

Mind that one who believes his own lies doesn't show any corporeal signs..

Also, I'd be surprised if someone telling white lies (or comfort lies) would show any signs of it..

look at the eye pupil when it gets bigger he is lying

That's BS...

https://www.youtube.com/watch?v=E3PAW7zjgPw

There is always a tell !

This method can also work.