basically they paste a full face onto another face. even if they do it convincingly there are still some issues around the exterior of the faces, so past the eyebrows etc. if it was a painting, look between the framing and the face. there will be something that doesn't look quite right. is this case, look for the hairline and the eyebrows. if something between those look a bit odd. it might be fake. i know it may seem vague but once you catch it, you'll understand what i mean.