This question is focused on the tournament rule, IPG 4.3 Unsporting Conduct — Improperly Determining a Winner.
Two players playing the Bant Nexus of Fate combo deck meet each other in a round. This deck wins by ticking up Teferi, Hero of Dominaria to the ultimate, exiling all the opponent's permanents, and then passing until the opponent loses to decking. The lock will beat just about anything. Anything, that is, except an opponent with their own copies of Nexus of Fate. Now they can also discard their Nexus to handsize and never deck out.
Let's assume Alice is executing the lock and Bob is discarding to hand size. Bob suggests a draw, since Alice is apparently incapable of winning. Alice says she has a win condition in her deck that will let her win regardless (say, Approach of the Second Sun), but she hasn't drawn it yet. Bob is reluctant to concede before seeing the win condition.
Are any of these allowed?
- Bob says "I'll concede if you show me a win condition". Alice searches her deck and shows Bob the Approach of the Second Sun. Bob concedes.
- Bob says "I'll concede if you show me a win condition". Alice gets her decklist slip and shows it to Bob (this actually gives up equity since Bob knows what to play around better now). Bob concedes.
- Bob says "I'll concede if you show me a win condition". Alice says she can't do this legally without violating the "improperly determining a winner" rule. Bob concedes anyway. Later he finds out Alice lied, and she doesn't have a win condition maindeck.
Finally: if #2 is legal but Alice doesn't want to do it (since it gives up equity), how can she legally demonstrate that she has a win condition in her deck?