You can catch cheaters by looking for unusual marks. These can be anything from a small blister to a pattern that looks like Braille. The most common method of marking is to use a daub. It is a liquid or powder that can be applied to the card with a finger, and then removed, leaving a mark on the back. These are usually very subtle, and can be difficult to detect, especially in a casino setting.
A pin or a blade can be used to mark a card. This is a more noticeable process, and can also be used on cards that are dealt face up. Note that in some casinos, this is illegal if the cards are marked to cheat.
If a casino knows that a player is using a daub on cards, they may not accept complaints. It’s important that players can spot these marks. They should also not be afraid to speak up when they suspect someone is cheating.
Remember that a deck with only a few marked cards can be very effective. This is because many cheaters simply mark high cards or Aces in order to give themselves an advantage over their opponents.