Magicians often use a prearranged or "stacked" deck of cards in order to perform an amazing feat. It is possible to detect that a stack is being used if the magician uses phrases like, "Hey, don't shuffle those!" and "Oh great -- now everything is ruined!" when you try and cut the cards.
The most common stack a magician uses is called the "Sly Stevens" stack. In this stack, the cards are arranged in order first by suit (in DitCHeS order – diamonds, clubs, hearts, spades) and then by value, two through ace. So the top of the deck would be the diamonds arranged two through ace, then the clubs, etc.
This order is deceptive in that it makes the deck look, at a casual glance, like it is brand new. And what could be more trustworthy than a new deck of cards?
However, a stacked deck makes an enormous number of stupefying effects possible.
For example, the magician might ask a spectator to cut the cards (but not complete the cut -- that would ruin the stack). The spectator then takes the card that was cut to and, without showing it to the magician, tells the magician what it is. The magician, knowing that the deck is stacked, can then magically reveal the identity of the next card down by taking the value of the stated card and subtracting one unless it is a two in which case the next card is an ace of the next suit in DitCHeS order! Magicians with poor memories may write the sequence of cards on a handy sheet of paper or have it tattooed to their forearm (because once a magician finds a good deck stack, it's good for life).