Josh:
Why do you ask the spectator that question? If they believe the cards are lost, having them say it doesn't make them believe it any more. However, it does, by negative implication, suggest a method -- "what if the cards aren't lost in the deck?" If they don't believe the cards are lost because you've done some of that hocus pocus, prestigiditation, "hands are quicker than your eyes" thing they will either lie (being polite while thinking the cards aren't lost) or challenge you.
If I was doing something like that, I would ask them "do you have any idea where the cards are in the deck?" The answer is going to be "no." My response would be "neither do I... that is what makes this interesting."
My script for one effect is as follows:
Me: Do you play cards?
Spectator: Occasionally.
Me: Do you always win?
Spectator: Not always.
Me: Do you cheat?
Spectator: No.
Me: That explains why you don't always win [this gets a laugh from the spectator and the audience].
Me: I want you to shuffle this deck till your heart's content.
Spectator [shuffles the deck]
Me: Are you done?
Spectator: Yes.
Me: [in a sincere voice] Is your heart content?
Spectator: [pauses, smiles and then says] Yes..
Me: Perfect. Perfect. I would like you to select one card, it doesn't matter which one you take.
Spectator: [selects one card]
Me: Can you tell everyone what you picked and show the audience your card.
Spectator: Ten of Diamond [as they show the card]
Me: Please put it back in the deck.
Spectator [puts it back in the deck]
Me: Can you hold the deck, I don't want to even touch the cards. Actually, put the deck pack in the box and hold on to it
The audience remembers that the spectator shuffled the deck, picked a card, put it back in and then put the deck in the box. They are thinking the trick is about to begin... but everything is already done. I don't ask "do you want to change your card" or "is this fair" and I don't make any statements "you could have picked any card you wanted but you freely selected one card" or "the deck is really shuffled so there is no way I could anyway influence your selection?" It just isn't necessary.
My script goes something like this.
Me: I need a couple cards for this, could you please call stop as run through the deck.
Participant: Stop
Me: Well you stopped me on an ace not to bad, and here are the other three. (I produce the 4 aces) Now you've found them once we'll see if lightning can strike twice and you can do it again. The idea is to track the aces, watch where they go in (I slowly and deliberately place the "Aces" into four positions in the deck) because in a moment you are going to follow your instincts and find the aces again. Do you think you can do it?
Participant: I don't know
(The participant then proceeds to "cut to the aces" with my guidance)
I never say "Do you believe that the cards are in the middle of the deck?" but I do everything but ask. With the stress on the participant I feel like I could easily ask them point blank if they'd like to shuffle, with their neck on the line I'm confident that they will decline.