AFAIK, this isn't something explicitly defined by the MTR or IPG documents. I think it comes down to the judge's assessment of the specific scenario.
In general, I wouldn't give much credence to pleasantries as a way to make that determination. It's too easy for players to have creative memories about end of game exchanges, and silly disputes like does “good game” mean “I concede” or “this has been a thrilling match so far.” For every player who says “You got me!” there's one that says “It looks like you got me!” and it seems insane to be resolving these disputes in a way that requires a linguistics degree.
The heuristic I've always used to determine if the game is over is as follows:
Has the (possibly) conceding player:
* Combined objects from different zones (e.g. scooped up lands with the contents of his hand)
* Revealed information from hidden zones (e.g. looked at the top card of his library, or flipped something exiled face down that he didn't know about)
* Removed counters from permanents
* Destroyed his record of life totals (usually by picking up his D20)
* Signed the match slip
Any of these actions to destroy the gamestate indicate to me that not only has the player acknowledged that the game is over, but they are past the point where they are double-checking that conclusion.
Additionally, if the (possibly) conceding player has taken verbal or physical actions which cause his opponent to do any of the above criteria and/or reveal private information, I consider that to be a concession as well. There may be exceptions if the non-conceding player has intentionally destroyed the gamestate in order to “lock in” a concession by a player who was still clearly in thought as to whether or not they were conceding.
Anything other than this I wouldn't consider to be a concession - nothing is preventing play from resuming and there's very little to be gained from engaging in a he said/she said debate. It is perfectly okay for a player to express that he thinks the game is over while still being in the process of thinking it through. Even beginning to gather permanents (without actually destroying the gamestate) can be a non-verbal communication that “I think this is over, but am moving slowly to give myself time to double-check.” Most players will scoop up their permanents as the conclusive indication that they have conceded the game, and most opponents will wait for that before clearing their own board. Trying to say a player has “locked in” their decision to concede before that is an arbitrary and potentially infuriating process.
Edited Glenn Fisher (April 29, 2014 07:17:02 PM)