Lie detection

"Oh what a tangled web we weave, when first we practice to deceive!" - Sir Walter Scott

How may we detect when a person is lying to us? It's not easy, that's why we get lied to so much. There are some indicators that may alert us to when a person is lying, and, while they are not totally reliable, they may cause us to stop and think about what the person is saying instead of just just believing them unconditionally.