Your Wisdom (Insight) check decides whether you can determine the true intentions of a creature, such as when searching out a lie or predicting someone's next move. Doing so involves gleaning clues from body language, speech habits, and changes in mannerisms.
I like the Insight skill. It's one my characters use frequently, to gain that extra edge in social encounters. It is however no surprise that, despite the fact that I envisage its use like a Sherlock-scanner of intent and micro-expression analysis, it is used about 95% of the time to tell if a NPC is lying to me.
This is not just the case with my own usage - as the PHB itself states, Insight is used to tell if a creature is lying. This turns the Insight skill from a DM perspective into a binary response roll: the player detects a lie or does not detect a lie. However, the way in which a DM conveys this information opens up space to interpretation.
