Am I obligated to identify cards owned by an opponent?
I too was unable to find anything in either the CR or MTR defining this, so I asked Matt Tabak, current Rules Manager of Magic and he said:


  I just assumed that was self-evident. Like, if you get up from the table, you should know which cards to take with you.


And a Level 2 Judge I asked said:


  From a practical standpoint, you’re going to be using different sleeves from your opponent 95% of the time. Even if the sleeves are the same, you have a right to your property, so you have the right to know which cards you own.