A combination of body language, and what the person says can give you the clues you need. None of these signs by itself proves that a person is not being truthful, but a combination of them can given you good reason to doubt.
First of all,
the actual words a person uses can tell you a great deal. Lengthy explanations to justify what they are saying may alert you that something is wrong. Unpracticed liars may give you a whole string of excuses just to make sure that you believe them. Sometimes they even look as though they don't believe what they are saying. You need to link what is said to body language before you can be sure.
Inability to make eye contact is the first sign, but some liars will stare hard at you so that you don't think they are afraid to meet your gaze. The eyes may wander all over the room looking for a way to change the subject. Sometimes a person who cannot look you in the eye is afraid that you will literally be able to see through them.
Other body language signals can be sweating, blushing, moving from one leg to the other or even jiggling one leg up and down if they are sitting. None of these are evidence on their own. You need to link them with other signals.
Your own instinct can be a reliable guide too. The brain gathers the visual information, assimilates what has been said and gives you that uncertain feeling that the person isn't being truthful. Don't ignore your instincts.
Gather your information and listen carefully to what the person says but remember that this alone doesn't prove dishonesty. To find out for certain you may have to do a bit more observation, or even ask directly.
Liars usually stare at you when lie.