With the release of EMN, GPT was change to Regular REL, that means no more game loss due to decklist problem. Although judges may ask players using decklist, I think some veteran players would try to cheat in some ways.
For example, in a GPT event Bob played 61 cards in his deck included one Duress but only 60 cards in his decklist without Duress. This one extra card would be very useful if there are lots of control or combo decks in this event, however, you would want to sideout this card if there are many aggro decks. Bob tried to cheat because he knew no any penalty in Regular REL.
Could you give me some advice in this situation, and is there any good way to prevent this situation happened?
(Sorry for my poor English, I try my best to explain the sitation)

Edited Che Wei Sung (Sept. 11, 2016 10:31:40 PM)

Bob can still get DQd if you determine he was purposely playing the card for this reason. 
If you're using decklists, then you should really be doing deck checks. 

Originally posted by Will Bumgardner:

Bob can still get DQd if you determine he was purposely playing the card for this reason. 
If you're using decklists, then you should really be doing deck checks. 

I would perform deck checks surely, but Bob may insist that he just forgot to rewrite a new decklist after adding one extra card. It is hard to determine he was purposely cheating or not, and we can not give him any penalty if we believe he just forgot.

If Bob “tried to cheat” by knowingly including this extra card in some matches, knowing that it was illegal to do so, this is a Serious Problem, and requires a Disqualification and Judge Center Investigation Report at any REL. (Of course, I was not at your event and didn't speak to your player, I can't tell you what to do in this situation, only cite relevant policy.) You can find more info in the Serious Problems section, towards the end of the JAR.

Ultimately, you need to investigate and make a decision. If you discover that the player is missing that singleton Duress during a Deck Check, you might talk to previous opponents. If any of them confirm that he did play a maindeck Duress, then you have that information for your investigation. You might also ask questions like when he chose to change his maindeck, and why. You could also check his deckbox, to see if the Duress and only the Duress is still there.

If you would believe bob had told the truth, then we do it as what JAR says about decklist problem, if you believe that he lied, then it's cheating. I don't think making hypothetical cases then dragging yourself in a dilemma would be wise and useful. We just deal with real situations and make decision's through evidences we receive, right?

And if you think cheating could be a problem, maybe regarding the prizes, you can always talk to the TO, see if it's nesessary to upgrade the tournament to Comp REL.

To second Siyang Li - cheating is usually associated with some kind of benefit/risk evaluation on the side of the potential cheater. I think that there are not many individuals who cheat just for fun and sports. If you believe that the possible benefit from cheating in the particular event would be higher, try to convince the TO to run it under Competitive REL. And remember that you should always have the option to not judge an event, especially if you feel that you would not have the necessary tools and support available. IPG is one of such tools.

Originally posted by Che Wei Sung:

It is hard to determine he was purposely cheating or not

It is indeed hard, but that's still the best deterrent to cheating: active judges watching games and investigating weird thing happening in their tournament.
We can create policies to lower the incentives to cheat, but no policy will ever replace the vigilant eye and mind of a judge when it comes to catching cheaters.

- Emilien