How to detect a lie?
Let's say that you have a friend who owes you some money. You have heard that they just got paid and you want to suggest that they pay you back. You try to be polite and ask them, "Did you get paid yet?" They answer, "Gee, not yet. The boss said maybe next week." and look to their right (your left). This would indicate that they are constructing or "making up" what their boss said. They are telling a lie.
Police often ask a suspect to describe someone in detail. If the suspect looks to their right (your left) they are generally constructing or "making up" the visual description. If they look to their left (your right) they are generally recalling the details from their visual memory.
So, in general, a glance to your left when observing a person indicates that they are creating or "lying" while a glance to your right signifies that they are remembering an actual event, or being "truthful."