Mfcttrf

I'm playing against an opponent who, for whatever reason, doesn't want to give away more information than they have to. (Perhaps they're trying to limit what is known about their strategy, their decklist, or their tells)

My opponent is one damage off lethal, but any creature will get him there. (Let's say I know he has a Might of the Masses in hand.) He plays a morph creature face down, swings (and casts his might of the Masses) and wins. Can I demand he prove the card had morph? Doing so gives me information in our next round that I shouldn't have, but it also proves that he didn't put just any card face down for 3

Is the only solution to get a judge involved? What if he already started scooping his cards back into his deck? Should I have asked a judge to verify that the facedown creature had morph before it resolved? If so, it seems unreasonable to call over a judge every time a morph creature is played. In this instance it's clear the creature was crucial in the moment, but in other cases the creature could have been on the field for several turns, and it didn't look so conspicuous when it was played.

Rule 707.9 says this:

If a face-down permanent moves from the battlefield to any other zone, its owner must reveal it to all players as they move it. If a face-down spell moves from the stack to any zone other than the battlefield, its owner must reveal it to all players as they move it. If a player leaves the game, all face-down permanents and spells owned by that player must be revealed to all players. At the end of each game, all face-down permanents and spells must be revealed to all players.

There is no way for a game to end without one of these things happening to each face down permanent, so every face down permanent must be revealed one way or another by the end of the game.

Failing to reveal a face down creature with Morph at the end of the game is treated as a Game Loss.

Per the MTG Infraction Procedure Guide:

3.2. Game Play Error — Hidden Card Error

Penalty

Warning

Definition

A player commits an error in the game that cannot be corrected by only publicly available information. It is not a Hidden Card Error if the opponent acknowledges the action or controls the continuous effect modifying the game rule that was violated. This infraction only applies when a card whose identity is known to only one player is in a hidden set of cards both beforeand after the error.

(...)

Upgrade

If a face-down card cast using a morph ability is discovered during the game to not have a morph ability, the penalty is a Game Loss. If the player has one or more cards with a morph ability in hand, has not added previously unknown cards to their hand since casting the card found in violation, and has discovered the error themselves, the upgrade does not apply and they may swap the card for a card with the morph ability in hand

This is reinforced by rule 707.9 of the Magic Comprehensive rules.

707.9. If a face-down permanent moves from the battlefield to any other zone, its owner must reveal it to all players as theymove it. If a face-down spell moves from the stack to any zone other than the battlefield, its owner must reveal it to all playersas theymove it. If a player leaves the game, all face-down permanents and spells owned by that player must be revealed to all players. At the end of each game, all face-down permanents and spells must be revealed to all players.