If you go to enough competitive events you will likely be deck checked at some point. If you haven’t, it’s nothing to worry about. Judges check decks periodically as part of their efforts to make sure that the event is fair. We want to make sure that not only are players not intentionally cheating in some manner, but that players cannot accidentally gain an advantage because of a deck problem.
One of the first things that happens in a check deck is the judge will clearly announce the deck check, tell the players being checked to stop, and then collect the two decks and sideboards. At this point the judge will ‘disappear’ with these decks, look at the cards and sleeves to make sure they are not marked and compare the deck list to the contents to the deck. Hopefully none of this is too surprising.
You may have noticed that the earlier paragraph started with ‘one of the first things.’ In reality the first thing that happens when judges are about to collect decks is that they watch. They watch the player that they are about to check how they are shuffling, to ensure that these decks are sufficiently randomized. As mentioned above, this is one of the things judges do to not only catch cheaters but to give peace of mind to the vast majority of players that do not cheat.
If there is a problem with either of the decks, sideboards or sleeves the judge will take the appropriate player aside, talk with them a bit, determine if the issue was intentional or not and issue a penalty if needed. In the majority of deck checks there are not any problems and the decks and sideboards are returned to their owners. At this point players should thoroughly reshuffle their decks. Judges generally sort the cards in the deck when comparing it to the deck list and do not randomize it when returning. Because players have to reshuffle extra time is given to complete that match. The standard practice is to give extra time equal to the amount used to check the decks plus three minutes to allow players to reshuffle.