Instead of looking for ways to handle that situation, you should focus on preventing it from happening in the first place.
First of all, I hardly do any card tricks, mainly because it seems like a trick. Now let me explain this. When someone sees you with a deck of cards they immediately think of card tricks (or Strip Poker... well, maybe that's just me). They believe that you are going to do a trick... and guess what, everybody knows a trick or two (specially card tricks!!!). And this sets you up for getting the line: "Oh, I know a trick too!".
Personally, I try to make people think that I have Psychic Powers
... that I do Real Magick. That's why I like to use ordinary items that can be found everywhere. I mostly do Money Magic
. (And some Mentalism
It can be helpful to get yourself new tricks, and learn new effects
. That way it's less likely that someone else would know it already.
Also, it's very important to create you own effects
and routines. That guarantees that nobody will know the trick.
You can also create variations
from current tricks that can even fool magicians who already know the original effect. (I do this all the time.)
Since I always do the effects in a Street Magic
style, and only to one or two people at a time, I don't ever get that line. By starting with something like: "Do you want to see some magic?" or "Let me show you something interesting." you set it up so that the focus is on YOU showing them something and not the other way around. It's about projecting authority
If they say: "I also know a trick." (Before I show them my effect.)
I can reply: "Who said I'm going to do a trick? I said Magic! Watch..." (Again taking control
of the situation... showing authority.)
After they actually see the effect they will not even bother showing me their Cereal Box Trick, or the same old card trick everyone knows.
Of course, if the person is also a magician (and I mean a good magician... or at least as good as you are) then it's different. Because they will actually know good tricks... they will most likely let you know. "I'm also a magician!", they might say. If after seeing your effect they tell you: "I also know some Magic.", then let them show you. You might end up exchanging tricks and tips
... and who knows... maybe you'll make a good friend.
Because I usually approach females to show my magic (stick?)... I don't really get anybody saying that they know the effect. (I'm not implying that women aren't good magicians... but rather that not many of them become magicians. Just like not many males become strippers. Oops! I think I made it worse but I'M NOT SEXIST... really.)
Anyway, my point is that if you show authority, confidence and take control of the situation this should never happen to you. And, that if you learn new effects, create variations of old effects, and create you own effects... it will never happen.
Oh, and freiburgflyer: that was hilarious!
P.S. If you do not agree with me then YOU ARE WRONG!!!
Imagine what a dull World it would be if everybody agreed on everything.
Hope at least one of you finds this useful. Then again, who wouldn't?