When the cheater picks up the cards and gets ready to shuffle, he makes sure to give himself good cards. The best cheats even manage to have magnificent flops. The cheat can be done distributing its (strong) cards from the bottom of the deck, while the others receive those above. But, with a little memory and usually, you can also cheat by giving the cards normally (example, you can shuffle only the middle of the deck by knowing the top Marked playing cards well.
Cards left under the table
One day, with friends, you can play without money and decided to allow any possible cheating and reveal at the end of the game what you did. There can be four and you all can have the same first reflex, leave one or two big cards on your lap and bring it out during the hands. In a casino, it would be very hard to do this because the players, the dealer and the floor are attentive. In a private part where it speaks, where a little alcohol circulates, it is doable.
By always playing with the same cards (especially when they are made of cardboard), signs of wear quickly appear on the back of certain cards. By knowing the package, you then know that the torn corner at the top left is on the 6 of hearts, while the large scratch in the middle is for the King of spades. Obviously, traces of cheating will not be easy to see, but when you know where to look, you only see that. For example, this could be a thin pencil line at an angle to indicate a big card like Ace, and the same lines in other places for other big cards.
Collusion at the table
What a defect in private parties and even in casinos with a lot of regulars! Collusion involves having two or more players who team up against you at their table. Partially private, this often results in friends who decide not to bet against each other, to follow the flop to the end to hit against a third player, etc. It is not a situation where they will always win, but they have an advantage over you by doing so.
Second player watching your guards
This type of cheating has already happened in big live tournaments. An eliminated player or a friend of a player stands behind you and codes the hand you have. Private poker games sometimes taking place on very small tables with the rest of the eliminated players circulating in the back, it is quite easy to get caught.