I have learned over the years (and had it confirmed through more recent published studies in trusted psychology and medical journals) that liars are not always spotted through body language. A person who is anxious for other reasons may avoid eye contact or look too long into your eyes. They may move their hands a lot. A person with a learning disorder, autistic disorder, and certain mental health disorders might do this too. So it's not a perfect method of detection. It can help catch many of them :) but not all.
The best method of detection is observing patterns over time. Some patterns will reveal the person is not intentionally lying, but has other issues--maybe a learning disorder to go along with it. But often the patterns in the rest of this answer reveal an habitual liar or a desperate person who might believe delusions.
Has the person told you something like "I will make sure you have a work schedule" and then not followed through multiple times over a period of weeks, even when you showed up and worked? They might have ADHD or another problem OR they might have been lying about their intentions in the first place.
Do other things they say not prove to be true...and when they are asked about it, they give an excuse that does NOT sound like they are sorry they spoke incorrectly or mistakenly? They might not have simply misunderstood or gotten excited and spoke without knowing--they may have been trying to lie!
Does the person keep saying things you know are not true at all, and then when offered corrective information (or asked a follow-up question that reveals you know it isn't true), retract the statement and state something else that also does not appear to be true? They might be lying, or they might believe it, and not know it's untrue. They may need some help, or they may have heard some gossip and thought it was true, or they may have high hopes without basis for the hopes.
Does the person brag about a time they fooled someone or "got them" to "do something"? They are a liar, and probably also a con artist, whether or not they participate in criminal activities. RUN.
I have lied about something before, but I am no good at it. People always ask me questions and even say this is why: Because I know I will know the truth.
(The problem with that is I may not know the truth or may have been told an incorrect thing...so.... I have learned to be specific about that too.)
My face is very expressive, and I also just can't help but blurt out the truth most of the time. I do keep secrets for people, but not the type that need reported.