Great question. The intention is for the player to never be 100% sure who is lying. The indicators mentioned on the TV are not 100% reliable, so you need to base your decisions off of multiple clues. For example, succubi view sexual topics differently from humans, it's supposed to be a sensitive topic but they might talk about sexual stuff a bit too casually... Also, they may be trying a little too hard to be polite. Another pretty big clue is sometimes they "break character" during dialogue because of being too horny.