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”

its depends on your own experience and understanding, bcz its true that when someone speaks lie his expression and gesture takes some changes, and if you can read them then you can know

His lips are moving.

This can only come from experience. Generally their facial expressions and demeanor will give them away but if it is a person who is an expert in lying, you will really have a hard time making that out. Another (impractical) way is to ask the person to take a lie detector test (which isn't fool proof either) A third way is to threaten the person with dire consequences if he/she doesn't speak the truth, but now we have crossed over to the realm of criminology. Be adventurous and explore more ways :P