Let say you are performing using the Invisible deck or a Svengali deck or whatever gimmicks or gaff you have in the deck you are using and just after the performance , the spectator wants to see the deck :O ... What do you do ?
This is a great topic to audience management. If you have great audience management, then they won't ask. I never have that problem anymore. What do I mean by audience management, well you need to learn how to present the trick in a way that they will just be stunned or speechless. I think if the trick is performed really well, then you don;t need to over sell it buy showing the deck. If the trick is not done so well, then the trick might look suspicious and they may want to take a second look.
Alright, there are several different answers that come to mind.
First and most importantly: Why are they asking to see the deck anyway? Yeah, there are some people who are just naturally like that. But these people are few and far between. I can't tell you the number of times I've seen someone say "Oh, he was just naturally intuitive" or something along those lines. And I try performing for them myself, and find that they asked to see their deck, but not mine. Truth is, if you think your spectator is just "like that", chances are, they're not, so think again, and think carefully: what are you doing?
Secondly, there are a lot of practical ways to deal with this. In no particular order, some general groupings would be:
a) Subtlety to prevent this from happening
b) Remove the gimmick
c) Switch the deck
d) Just show them the deck anyway
a) would include things like flashing the deck, handling the deck openly, starting with an effect which uses a normal deck and letting them examining it, etc.
b) would include things like palming a double backer out, etc
c) would include an in the open deck switch, the use of a deck shell, or even just putting your deck away immediately (but subtly!) after use
d) may sound counterintuitive, but you'd be surprised how easily a double backer etc goes unnoticed. The more hesitant you are, the harder they'll look. Often they don't really want to examine the deck - it's just their first reaction (refer to note one above).