Usually the defining characteristic of a psychopath is an inability to feel shame, remorse, or empathy. If you catch someone with narcissistic personality disorder in a lie or situation where they've hurt someone and call them out on it, the response is usually bluster, a frantic attempt to shore up ego. If you catch someone with borderline personality disorder or just plain deep insecurity, they'll often deflate and get downcast. Catch a secure person and they'll apologize and try to make things right. Catch a psychopath and they'll react rationally to further their own interests - they completely lack an emotional response to hurting people.