I was doing Fearsons floating ciggarette and sombody said "Hey you have a giant magnet in your mouth!" "And a needle is stuck inside of the ciggarette!"
When I once did the tenkai vanish where the card goes up your nose and out of your mouth sombody said "You just had the card between your fingers and you bent your fist in and out!"
And The Dime and Penny Trick where the dime travels from hand to hand, A girl in my class said "You had another dime in your hand. You just put the penny on top of the dime in one hand then you closed you hands and the penny was still on top of the dime and you opened your other hand to reveal the duplicate dime!!!!" Of course I said "No thats not how its done" And she kept argueing with me. I hate her!
Denying is usually a dead giveaway unless you are prepared to prove them wrong. You're better off if you never confirm or deny when someone says how they think it's done. When people try and tell me how I just did that trick, I usually just smile with amazement and say something like, "Wow! That's an interesting theory. Maybe I'll try it that way."....or something similar. It's easy to deny an incorrect theory. But there is a different attitude from people (magicians) when they try to deny a correct method for doing a trick. This is just like those people who, hypothetically, have a secret...and you or someone else keeps trying to guess what it is. They keep saying no, no, no (that's not it). But when you get it or get close, they change their reply to "I'm not going to tell you". It's a dead giveaway.
Here's what you have to remember when someone is claiming to know and telling you how they think you did the trick. Even if they are guessing correctly, they are only guessing. They don't know. Let them believe what they want to without any influence from you. You don't confirm, you don't deny and they'll never know.