Anyone of a Tier lower than you suffers +2 Complication to lie to you. If you are currently casing and they are somehow involved, they simply cannot lie to you at all. The Storyguide should also notify you when a lie has been told, though it does not automatically reveal the truth, and also reveals nebulous truth, such as “he has been convinced this is true” — which should always be a means for characters to pursue the person who manipulated the target in the first place.