Simple question but I don't have an example case.

As a judge, are we allowed to say explicitly to a player something like, “I suspect a cheat here but can't find a proof for now. So, I'm watching you.”

Are we allowed to? Well, yeah, nothing prevents that … but I don't recommend it. If you suspect cheating but can't be certain, let them think they've “got you fooled”, and continue to watch. If you let them think they got away with something, they're more likely to try again; if they think you're watching closely, they'll be more careful.

d:^D

Generally speaking, if we suspect a cheat and we are investigating accordingly, telling the player might scare them into stopping the behaviour for the duration of our event, but runs a high risk of only stopping the behaviour when they think they are being watched (which was probably true before, but now they are extra vigilant for our prying eyes!). Certainly, the player will likely go away and cheat at other events and the relevant people would never have known.

Of course, if the player isn't cheating and we tell them this then that is going to create some pretty awful feel-bads when we go over and tell an innocent player that they look suspicious and that we're watching them; I know that would make me uncomfortable were I told someone was watching me!

Keep in mind that whilst we may have the title of judge, we are not bound by the same rules as mere mortals (sorry, couldn't resist! We are, as far as I have been made aware, merely mortal) most courts of law and we do not require proof beyond all reasonable doubt. If you have good reason to believe that this player is up to no good (which it sounds like you would in this scenario) and that the integrity of your event might be compromised, that is justification enough to disqualify a player and you can explain this to them when you are disqualifying them.

At a minimum, this would seem like pretty poor customer service. It's a
straight accusation while admitting you don't have enough proof. It seems
like there's a lot of ways that can go bad and almost no way that can go
good.

If you suspect this person of cheating, saying something will give away the
ruse and that player will likely stop. If your goal is to catch them and
DQ them, then you just blew your cover. If your goal is to discourage them
from continuing to do it, then I think there's a lot more polite ways to go
about it. “Hey I noticed your shuffles don't always catch the top card.
Be sure you're randomizing all of the cards equally.” or “Be sure that
you're not holding the deck so sideways when you shuffle. We just want to
make sure no one sees the bottom cards.” or “Did you tell him he could
search for a basic when you Path'd his dude?” You can even misdirect their
thinking by not mentioning what you think they're doing (ie if you think
they're shuffle cheating, double check if they gained life from lifelink).
I think all of those statements prove to the player that you're actually
watching for things and could discourage them from continuing.

Short answer: Yes, you are allowed to say that.

Long answer: No, you really shouldn't, for a couple reasons.

1) You're tipping your hand. As Scott said above, if the player knows you're watching, they'll be less likely to cheat, and then you won't have any proof and that player won't be caught and dealt with appropriately. It's better to catch cheaters and get the cheat on their record permanently (yes, you have a permanent record with the DCI) than it is to stop them from cheating at one event (and then go back to cheating at the next one).

2) That phrase (at least in English; I notice you're from Indonesia so maybe the phrase in your language is different) is very aggressive. If a judge said that to me, I'd probably report that to the TO and say that the judge is biased and I don't want that judge staffed anymore. TOs listen to that sort of feedback, because TOs want feet in the door and butts in seats and will hire whatever judge will get them that, more than they want any particular judge to run their events. So if you come off as aggressive towards one or a small group of people, and drive those people away from the store, then you could jeopardize your chance to be staffed at that store in the future.

3) It could be a simple mistake. I was judging a tournament a couple months ago where I noticed a player shuffling in a way that looked a lot like cheating. So I watched him to check for consistency, and I deckchecked him to check for deck ordering, and I watched some of his games to see if he drew above average (he was at the top tables), and I was unable to come up with anything definitive (he wasn't drawing particularly above average, when I deckchecked him his deck wasn't particularly well-ordered, etc). So after one game, I pulled him aside and I said “I've been watching you during the event…” and I mentioned to him the issue I saw. The player was completely unaware of the issue, and after talking to him I noticed he changed his technique.

Edited Lyle Waldman (Oct. 12, 2017 12:45:50 PM)