When the light disagrees

Lighting is a stricter test than anatomy because a scene has one physical light setup, and every object in it must obey that setup. If a single light source casts shadows in two different directions, or if a reflection shows a scene that is not actually present, the image is internally inconsistent. The same applies to materials: glass, metal, and water have predictable behavior, and surfaces that bend light or reflect it impossibly are strong cues. These inconsistencies are harder to explain away as camera error than a strange hand.