Tournament rule 411.3

Random deck checks should occur at the start of a round, after players have randomized and presented their decks, but before opening hands are drawn and should verify all elements of the registered decks.

• 411.3.a A deck check can be performed after the mulligan procedure is complete as long as play has not begun and each player’s…
• 411.3.b If a deck check is performed within a match, judges should grant a time extension to that match sufficient to cover…

Official Tournament Rules