You can never prove someone is lying just by looking at non-verbal cues -- but they can let you know when something might be off. David DeSteno, a professor of psychology at Northeastern University, conducted a thorough study and found four non-verbal indicators you might want to look for.
Photo by Dyanna Hyde
The study is fascinating and the full video below is worth watching, but here are the TL;DR results, There are four body language cues to signal that a person may not be entirely honest: touching hands, touching the face, crossing arms, and leaning away. Individually, they haven't proven to be effective indicators of a liar, but combined together, they're a bit more useful.
DeSteno states in his paper, the video and an article that this may not work when someone knows their body language is being observed for a tell -- these are unintentional cues given off when a person is trying to hide the truth.
How to Tell if Someone is Lying [Harvard Business Review]