Tha “niggle” @definitive spoke of is the best way to tell if someone is lying. It’s very hard to use it properly, too, because it has to be an objective niggle, not one colored by your desires and prejudices.
I think the data show that there are no standard rules about lying. Some people lie staring in your eyes and some people tell the truth dodging your gaze. You can’t trust these tells, unless you’ve got a lot of experience with someone—the way a gambler does in studying his opponents for signs they are bluffing. Over time, you come to know, but there is a different tell for every person. You can’t assume it’s always the same.
In absence of surefire tells, you need your niggle detector. What’s that? That’s your idea of how the world and people work and what they are likely to do in various situations. We call this a model of life. It’s your current understanding of human relationships and natural relationships.
Obviously, a good model of this kind takes decades to build. Young people, because their models are less sophisticated because they have less data to build the model on, will have a harder time detecting lies. However, your niggle detector can still help. You try to match up the potential liars behavior with the behavior you would expect if they weren’t lying. If there’s a discrepancy, then you start to think they could be lying. Some people also call this the “smell test.”
When your world model is unsophisticated, you will detect lies where they don’t exist and miss lies that do exist much more often than a person with a sophisticated model. It can’t be helped. You do the best you can. However, if you’ve got a niggling about a lie, you should at least gather more information to see if you can find evidence that something is wrong. I would act as if I believed the person if I had no other evidence other than my niggle.
The tricky part is that your niggle detector can be confused by your own bias. You want to believe what you think, and that can lead you astray. You’ve got to do your best to look at the situation as if you were some external, neutral person. It’s the best you can do until you find the person’s “tell.”