Other ways, besides lying yourself, that you could know a person was a good lie detector: * You've seen them catch others in a lie * You've heard stories abou

Other ways, besides lying yourself, that you could know a person was a good lie detector: * You've seen them catch others in a lie * You've heard stories about the ability 2nd hand, either from people who have been caught themselves or from people who have seen others get caught. In addition, there are many fuzzier, less reliable types of evidence: * Somebody has so much knowledge and experience about something that you can safely assume irregularities would stand out to them. * They just give off that vibe: soul-piercing gaze, etc. * They tell you. So logically, the question makes about as much sense as asking how you could know someone was a good cook if you've never tasted their food. And not only that, getting caught in a lie by someone isn't even necessarily good evidence of their lie detection abilities. Maybe you are just a terrible liar. Maybe they got lucky one time.