What do call a person who sees that the rules of game are followed?

There are a few ways to describe someone who ensures the rules of a game are followed, depending on the context:

Formal:

* Referee: This is the most common term used in sports.

* Umpire: This term is used in some sports, especially baseball and cricket.

* Judge: Used in some competitive activities, like dance or chess.

* Moderator: This term is used in online games or forums to ensure proper conduct.

* Official: This is a general term that encompasses all of the above.

Informal:

* Rule enforcer: This describes someone who strictly ensures the rules are followed.

* Game master: This term is often used in tabletop role-playing games.

* Watcher: This is a simple and neutral term.

The best term to use will depend on the specific game and context.