I'm glad to see some attention being paid to playing cards, one of the oldest fundamental artifacts of gambling. In the last three to five decades the introduction of custom produced cards has made cards attributable to a specific club - but these cards also introduced a great number of previously not offered variations that a casino could choose from. The best sources for understanding the variations offered are the manufacturer's catalogs - if you can find them. With the expansion of the use of the internet for sales, many of the manufacturers would have a relatively good description of the cards and choices offered. Unfortunately some recent consolidations of manufacturing companies have reduced the number of websites that can be found as of this date.

If you are still willing to dive into this rather deep pool of collecting varieties - but very interesting - consider just a few of the differences found on decks:
Card size - Back color - Back design - Ad design and shape (logo) - Index size - Index position - Suit valve marks - Deck finishes - Cancellations - Special work - and more!!

I personally scan - Ace of Spades - King of hearts - Joker - Manufacturers card - Advertising card - Box design - and any other cards that are not generic to the usual cards found in the deck. Remember that used decks are not always returned to their original boxes by the people who process used decks for re-sale.

If you are really into details - take a shot at collecting "Special work" (marked cards) - and you can add a totally new set of additional things to document.

Do you want to collect full decks and document the representative examples - have fun!