This has never happened to me before, but if it did:
"The Joker? All right, please note that my hand is empty as I take out the tuck box. Go ahead and open it; inside should be three cards. Two of them are ad cards, but go ahead and show the other one around -- ladies and gentleman, the Joker."
I may or may not forget to tell them that I always leave the ad cards and one Joker (the other one's usually serving as page marker in some book or another) in the tuck case.