I don't know why you choose this topic to ask that question but here's what I know.
There are special decks that look like normal decks at a glance but when the back of the deck is examined tiny discrepancies can be found in the artwork, the discrepancies tell what suit and the number of the card.
The deck I saw had four small peacocks in the corners of the artwork. The peacocks had their tails spread and each had 14 feathers(one for each number in the deck). All the feathers of every bird except for one were white. One feather on one peacock was blue.
So for instance if the card was the 4 of Spades the upper right peacock would have it's 4th feather filled in with blue. If it was the King of Hearts it would be the 14th feather on the lower right.
Hope that was concise enough to help.