Well. A LIER you can easily detect just by looking at them, Lier means to rest or recline to a specific spot. Example being a cat or animal sleeping or usually in the same spot a lot resting.
Now if you're meaning a Liar, there is no one way to determine if they are lying. They're their own people and each person went through their own thing in life. Example being if someone talks about a tragic incident, it could be true. Or a example of if someone been bullied or abused. It could be true.
Generally, 90% of things people say could be true, but if they say they fly on a magic broomstick then you can rule it out that they are Infact a bit out of it. But respectively you have to consider most things told to you as true unless proven otherwise. It's also just to avoid unnecessary problems, for instance if someone said they was abused, but you don't believe them and it turns out true, then you end up looking like the prick.