Mostly it seems to be about sensitivity in picking up non-verbal clues given off by a person who is lying: body language, facial tics, changes in the pupil of the eye, blink rate and so on.

Such things are not under conscious control and an experienced person is able to read them in a subject.

Funnily enough, it is quite common for the observers to themselves be unaware of what they are interpreting, so one gets the idea of a "gut instinct", "I knew he was a wrong 'un", and even though experienced, an observer may not know exactly what it is they are looking at which gives the impression of mendacity.

Successful con-artists try very hard to control these autonomic responses and sometimes they are very good at it. Politicians, too... but that is another debate!