Watch their body language.Often the liar will not look directly at you or make excuses not to meet you.
Also a guilty person gets defensive. An innocent person will often go on the offensive.A liar is uncomfortable facing his questioner/accuser and may turn his head or body away. A liar might unconsciously place objects (book, coffee cup, etc.) between themselves and you.
A liar will use your words to make answer a question. When asked, “Did you eat the last bit of chocolate?” The liar answers, “No, I did not eat the last bit of chocolate”