You show a blank card that is attached to a lanyard around your neck. You ask a spectator to choose, and sign, any random playing card from a deck. The spectator then places their signed card inside a card box. You tell them that you will make their signed card vanish. However, when you open the card box there is still a card inside.
You then explain that their card did in fact vanish and you show them that the card inside the card box is blank. You then remind the spectator that at the beginning you showed them a blank card attached to the lanyard around your neck. When you turn this card over, it is their signed card. Everything is examinable.