Marking Bicycle ESP cards

Hello t11,
I recently got 25 ESP cards with Bicycle backs. They are not marked (or I don't see the mark. They didn't come with any instructions) so I decided to mark them myself. I know that most of the time spectators won't look for a marking system on your cards, but what is the most efficient and hidden way to mark ESP cards? I already have an idea, but if there is an efficient way you know of I would be grateful if you could share it with me.
Kindest,
Adi
 
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