The marked cards refers to the playing cards that are with invisible ink marks on the back. Usually the IR or UV contact lenses and sunglasses can see the invisible marks clearly, and that's what people use in the poker games for cheating.
Firstly, you can just put the doubted marked decks with the clean playing cards. If they have color difference. At least one deck of them has problems.
Secondly, put the poker decks under the sunshine. There would be marks on the marked deck and you will see it with the sunshine.
Thirdly, Money detector with purple light can check the invisible ink marks. You can just check the decks with money detector if you doubt about the decks.
Just try these methods and you can check the marked decks for sure if they are marked.