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."
A typical approach in this magic trick involves dividing the original card into smaller sections, giving one section to an audience member as proof of ownership, and then secretly disposing of the remaining pieces; when the lemon is opened, the card inside will be found to remain intact with a torn corner that corresponds with the "proof."