If someone surrenders can I check their graveyard?
Once a player concedes they aren't required to show you their graveyard, as at the same time the player leaves the game so does their graveyard (rule 800.4a.).
While rule 400.2. means that you can look at their graveyard at any time because it's a public zone, rulings relating to 104.3a. mean that a player can concede prior to actually showing you the graveyard after you have requested to see it.
However, as the cards that are being milled are entering a public zone it is entirely within your rights to ask to view them as they enter the graveyard, again the player can concede at this point, but you may be more likely to see what's being milled if you view the cards at the same time as the other player.
Related rules:

104.3a. A player can concede the game at any time. A player who concedes leaves the game immediately. He or she loses the game.
400.2. Public zones are zones in which all players can see the cards’ faces, except for those cards that some rule or effect specifically allow to be face down. Graveyard, battlefield, stack, exile, ante, and command are public zones. Hidden zones are zones in which not all players can be expected to see the cards’ faces. Library and hand are hidden zones, even if all the cards in one such zone happen to be revealed.
800.4a. When a player leaves the game, all objects (see rule 109) owned by that player leave the game, any effects which give that player control of any objects or players end, and all spells and abilities controlled by that player on the stack cease to exist. Then, if there are any objects still controlled by that player, those objects are exiled. This is not a state-based action. It happens as soon as the player leaves the game. If the player who left the game had priority at the time he or she left, priority passes to the next player in turn order who’s still in the game.