I am unsure if this works on every zombie map, but I think it does. It definitely works on Der Riese.

You can't normally see how many kills/downs/headshots that you have gotten in solo zombie mode, but Unmazing is here to help:

Start a split screen 2 player game. Quit that game, back out until you can choose a solo game and do so.

Now you can hit the 'back' button to review kills/headshots at any time during the game.