How do I see what level zombies I have killed?

Your personal information page in the game will tell you how many zombies / enemies you have killed but not the higher level you have hit so far. The easiest way to find out is to tap on the zombies in the world map and see if you can attack them. The game will only let you attack zombies of the next level after the one you have already defeated or below.

For example if you have defeated level 9 zombies then you can attack 10 or below. It will not let you attack 11 yet.