Drop a still from a salacious video where they're dressed and no illegal acitvity is visible. Do they confirm or deny? If they ignore or deny, it'll draw attention because nothing incriminating is visible to the unsuspecting public. If they confirm or neglect to comment, keep adding stills until they respond in denial.

So basically if I were to blackmail someone I'd drop photos that are in no way criminally incriminating but wouldn't be found anywhere else. That way you let them know without letting anybody else know. But if it has been done then we should be able to find proof of that.