One good way to tell if someone is lying is if they are looking to their left, your right. If they’re looking to the right (your left), they are recalling a memory, a smell, something. They remember it. Looking the other way, they are making it up as they go. However, this can be avoided by a good liar. They’ll have trained themselves not to look to their left when lying.
Another way to tell if someone is lying is to see if they mix up the details. Many times, when a liar doesn’t know the story well enough, they will say something wrong if pressed to remember a specific lie. Going on this same note, if someone tells you a story with so many details it seems impossible they remembered it all, they’re probably lying to you.