This trick, from Presto's secret stash, won him high honors at the 2006 annual meeting of the Global Genetic Scientists Consortium.
The spectator is shown that a deck has been well shuffled and is graciously allowed to choose a card. After the card is shown to the audience, it is lost in the deck.
The magician puts the deck away and hands the spectator a cotton swab with instructions that the swab should be rubbed gently across the inside of the (spectator's) cheek. The swab is sent to a genetics lab where cells it picked up from the spectator's cheek are used for gene sequencing.
When the spectator's genome comes back from the lab, it is taken from its sealed envelope by the spectator. The magician points out that there is so much information in someone's genes that the sequence is essentially random. Even so, the spectator's choice of card was fated -- in fact, it's embedded in their genes.
After a quick look, the magician says, "Ah ha!" and points to a location in the genetic sequence. There, amid billions of letters indicating the spectator's genetic makeup, are a pair of letters together that indicate a card -- "AC," the Ace of Clubs.
Sure enough, this is the spectator's card.
The magician uses a force to make the spectator choose a specific card (e.g., "Here, how about this one?") The genetic sequencing is all completely legitimate.
What many people don't realize is that genetic sequences are coded using only four letters -- "C," "T," "G," and "A." Given that there are billions and billions (and billions) of these letters all jumbled about in a genetic map, there is an excellent chance that somewhere in there an "A" and "C" will be next to each other in that order. A little practice will aid the magician in scanning the genetic map quickly to find the lucky pairing, avoiding the many hours necessary for a detailed look.
If, during the scan for the magic letters, you happen to notice some genetic flaw (a propensity for baldness, hereditary anemia, etc.), be sure to keep it in mind if you intend to do any mentalism later in your act.
Note: This trick's secret is not in any way related to the secret behind "Cut and Informed," in which a dying person chooses a card and its identity is found in the form of scar tissue days later during the spectator's autopsy.