You probably have an instinctive idea of when someone is lying to you. But, despite what poker shows and police procedurals would have you believe, it is generally difficult to spot deception. Research has debunked many of the stereotypical signs we think people display when they're bending the truth—fidgeting, averted eye gaze and so on. Overemphasizing these unreliable cues can actually make it harder to spot lies, especially if the liar is well practiced.
A study published in Plos One found that self-reported "good liars" tell small, simple lies and try to avoid adding extra details to their stories. They also tend to repeat their lies more often than honest individuals do, a process known as ACID (accountability, consistency, integrity and detail). This type of behavior can be hard to detect in person, but it is easier to identify over the phone or via email.
In addition to avoiding specifics and repetition, liars may also give short, uninformative answers or try to avoid answering your question at all. These are all red flags. However, it's important to remember that not all of these hints are accurate or reliable—particularly when the person is nervous or trying to conceal their true emotions.
It's best to get a baseline for the person and pay attention to their behavior when they are being truthful. If you notice a sudden change, that's a good indication that something is up. Then, ask them to repeat their story and look for discrepancies in tone, word choice and facial expressions.