I'll get the secret out of the way here. This routine uses a stacked and marked deck. With that information you will be able to follow along with what's going on as I describe the effect. If you don't know a stack, then The Code deck, by Andy Nyman, would be ideal for this effect as well.
The trick is a little to me-centric for my tastes these days, but it's a very fun trick to perform and it might be of some interest to you.
You're talking with your friend and you mention some things you're working on in regards to magic and mindreading. At some point you bring up the concept of "using my five senses to mimic a sixth sense." You offer to demonstrate some of the exercises you've been doing to work on increasing your sensitivity to sensory input.
There is a deck of cards on the table.
You go into another room or stand in the corner Blair Witch-style. You ask your friend to take the deck, cut a portion of cards off of it, and toss those cards under one of the couch cushions. You then have her cut off another portion of cards and set it on the table. "Take the card on top of what's left in your hand—the card you just cut to—and put it in your shirt-pocket or in your bra or somewhere close to you where I can't see it." You then instruct her to place the cards that are left into the card box.
You turn around or come back in the room.
You take a seat on the couch, look up for a moment as if thinking about something, then write a few words on a piece of paper, fold it up, and place the note on the couch.
"Okay," you say, "the first sense you learn to exercise is vision. There are a number of physical exercises for the eyes. And there are also techniques that you practice to take in large quantities of information in a brief amount of time. Like in Rain Man when Dustin Hoffman counts the toothpicks."
Your 26-year-old friend has no idea what you're talking about and makes you feel old.
"Anyways," you say, "where is the second packet you cut off?" She points to the packet on the table. You ask her to pick it up and hold it between her thumb and first finger. You walk around the packet from a distance giving it a brief look all around.
"I think there are 17 cards there," you say.
She counts the cards and there are 17.
You talk for a moment about exercising your hearing. You ask the spectator to shake the card case with its cards inside. You listen intently. "I think there are 11...no... 10 cards in there." She counts the cards and you are correct.
"Knowing how many cards are in both of those packets, it would be pretty easy for me to now tell you how many are under that cushion that I was sitting on a moment ago. I would just subtract the number of cards in those piles plus the one in your pocket from 52. Yes, it would be easy for me to tell you that now when we've counted these packets. But when I first came back here, and I had no information to go on—I didn't know if you had put 2 cards under the cushion or 40—I made a note on that piece of paper. Go check it."
She opens the note and it says, I'm sitting on 24 cards. She counts the cards under the cushion just to double check, and you were correct from the start.
You have her reassemble the deck. With your back turned she removes the card from her pocket, reminds herself of its identity, and shuffles it into the deck.
You spread the deck face-down on the table. You subtly sniff your way around the cards. Just short, rapid inhales through your nose, don't be gross about it. You start pushing cards away until you've narrowed it down to one card. You ask what card she had in her pocket. She says the three of hearts, you turn the card over and it's the three of hearts.
As I said, it should be obvious, but briefly here's what's happening:
1. When you come back into the room you note the top card of the stack on the table. One less than that cards stack number is how many cards are under the cushion. You write that on the piece of paper.
2. When you are circling the packet the spectator holds to test your vision, you note the bottom card of that packet. That's the key that unlocks everything else. You now know how many cards are in the "visual" packet (that card's stack number minus the number of cards under the couch cushion). You know the card that's in her pocket (the next one in the stack after the one you just glimpsed). And you know how many cards are in the "audio" packet. (Subtract the stack position of the card you just glimpsed from 51. (51 because one card is in her pocket.))
The rest is just acting and having fun.
I performed this effect probably 6 or so times before I retired it. And I think every time I did, the spectator asked about taste. Now, you could routine in another effect where you figure something out via taste, but that just makes it slightly too long, in my opinion. Instead I suggest you have some sort of joke or bit planned since it's probably going to come up.
I hesitate to say what I did because it's not a good idea for most performers, but it worked for me and the people I performed for. They would say, "What about taste? Do you use that sense in magic?"
"Yes, indeed I do. Here... I want you to think of any emotion at all. And don't just think of the emotion, but also think of a time you felt it strongly. Do you have an emotion in mind? Concentrate... concentrate... I want to see if I can pick up on the emotion you're feeling. You're concentrating, yes?"
They'd agree and I would grab their head in my hands and quickly and sloppily lick up the side of their face like a dog.
As they said, "Ugh!" and wiped my saliva off with their sleeve, I would click my tongue against the roof of my mouth as if I was evaluating the taste of a wine or something.
"Yes... yes... it's coming through. The emotion is very strong. You're...hmmm... yes, you're totally disgusted!"
They would have to admit my powers proved correct.
I only did this with old friends (both male and female) or newer friends who had indicated they didn't have an issue with me licking other parts of their body (if you catch my drift), so I knew it wasn't a complete invasion of their personal space. Is it gross? Yes, that's the idea. But it's more funny than gross with a close friend, so long as you have good oral hygiene. "Well, Andy, I don't think it's funny at all. If you licked my face we would have a serious problem." Well, guess what, you fucking lame-o, YOU'RE NOT THE TYPE OF PERSON I WOULD HANG OUT WITH. So it's not an issue.