pf2e.fyi
From Pathfinder Lost Omens Pathfinder Society Guide, which isn't enabled.

Marked Playing Cards

ITEM 0
Price 1 gp
Usage held in 2 hands
Bulk —

A deck of marked playing cards appears to be an ordinary deck upon initial inspection but bears specific folds, colors, scratches, or other markings on each card that allow an informed user to know what's on the other side of a card. A deck of marked cards grants you a +1 item bonus to Games Lore checks or other relevant checks to gamble with the cards. You can determine that an unattended deck of cards is marked with a successful DC 20 Perception check. Determining that a deck is marked during play is more difficult and is typically determined by a Perception check against the dealer's Games Lore or Thievery DC.

Pathfinder Lost Omens Pathfinder Society GuideOGL