Answer:
"Are you a cop?" will often suffice, if you have ever watched the show "Cops" the fake prostitutes and fake johns always reply to this question with retorts like: "Are you kidding?" or "Ha! What? Are you a cop?" So you see, they are not actually saying "no". It would not legally be considered "entrapment" if they did say no, but officers of the law cannot lie about their status. Please note: this does not apply to federal agents, or people working for the D.E.A.