Cheaters gonna cheat.
If you suspect a person/team of cheating you can do the following:
1. Get a copy of the team roster/game report. For league and cup games- The home team brings 3 copies of this report. Any players or coaches not on the printed roster can be written in (on all 3 copies). I believe that state cup does not allow any guest players that are not on the printed roster.
The roster/game report does not get submitted anywhere. The referee takes all 3 copies at the start of the game. The write down goals and cards as required during the game on 1 copy. At the end of the game- the write goals and cards on the other 2 copies. The referee keeps 1 copy and gives a copy to each of the team managers/coaches.
The referee should check that all player cards match the names on the roster. They usually don't look very closely at pictures or birthdates.
If you suspect a player was playing that shouldn't have been - you contact the league and provide them with a copy of your game report. They will then investigate for any wrong doing.
2. Same thing with a coach. If you suspect that a coach is there that shouldn't be there- you probably want to have your coach or team manager request to see the coaches card. You'll need to do this right before the game, at halftime or immediately after the game.
The referee gives all cards back to the team at the conclusion of the game.
The system is set up with the expectation that there is a certain level of trust by teams. But if they want to bend the rules, it's not very hard to do.