A common method is to tear the original card into sixths or eighths, give one piece to the audience member to hold as a "receipt," and vanish the other pieces; when the lemon is opened, the card inside is shown to be intact except for a torn corner exactly matching the "receipt."
One common technique involves dividing the original card into smaller sections, giving one section to an audience member as evidence, and then concealing the remaining parts; upon opening the lemon, the card within is discovered to be undamaged, except for a torn edge that corresponds with the "evidence."