There’s been a lot of speculation and worry about a rise in Chinese counterfeit cards recently.
First of all, don’t panic. Despite some concerns, this is not a widespread problem, and most judges will never encounter a counterfeit card. You’re more likely to come across a player who accidentally left a sharpied proxy in his deck! There’s no need to go on a witch hunt or whip out a jeweler’s loupe during a deckcheck. Just do what you’re always doing, and if you see a card that doesn’t quite match, or looks out of the ordinary, follow up with the player to try to figure out what’s going on.
If the player knew they were playing with counterfeits, things are pretty simple. They’re out of the tournament. Make sure you get email confirmation from them in their DQ statement in case Wizards wants to follow up.
If they aren’t aware that they have a counterfeit card, remember that they had no idea that they were doing anything wrong and now have the double-whammy of a Game Loss and knowledge that a card they bought/traded for isn’t a real card. Be understanding of their situation. Obviously they have to replace the card (and possibly with a basic land), but give them a little time to do so, and be sympathetic to their plight. They’re a victim here. Don’t try to take the card from them, but do encourage them to provide as much information as possible about where the card came from. Getting that (and sending it along to email@example.com) will help Wizards track down the sources and hopefully prevent the player from being taken advantage of in the future. Exercise excellent customer service here.
Finally, a cautionary tale. At Pro Tour Honolulu, the judges came across a Hypnotic Specter in one player’s deck that was… off. It was too glossy, felt wrong. The light test suggested a very good counterfeit, so the card was confiscated (something we wouldn’t do nowadays, but was the practice back then). There was still quite a bit of debate about the card, so the staff finally decided to do the rip test and tore the card in half. There was a black stripe inside. Unexpected. It turns out that there’s a very small experimental run (the Alternate Fourth Edition set) out there that were different, but were totally legal cards. Fortunately, it wasn’t a priceless collectible (and the judges took care of replacing the player’s card), but don’t just default assume a card is counterfeit. Work with the players to be sure!