Look for any unusual marks on the back of the card to catch a cheater. They can range from a small blister up to a pattern which looks like Braille. The most common way to mark a card is with a daub. This substance is usually liquid or a powder, which can be applied using a finger. Then, the substance can be removed from the card leaving an impression. These are usually subtle and can be hard to detect, particularly in a casino environment.
Another method of marking a card is by cutting it or pricking it with a pin. This is a more noticeable process, and can also be used on cards that are dealt face up. This can be illegal if done to cheat in some casinos.
In fact, if a casino is aware that a player has been using a daub to mark cards, it may not welcome complaints about it. It is important that players are able to identify these marks, and not be afraid to speak out if they suspect a cheater.
Remember that a deck with only a few marked cards can be very effective. Many cheaters will mark Aces and high cards in order to gain an edge over their opponents.