Too Many PLayers on Field

[BSBAL18]

HighSchool:

So a team can play with 8 on the field, but not 10. Before each inning i always do a quick could to make sure there are 8 or 9 on the field. My question is, what happens if i could 10? Is it my job to bring that up, as the umpire, or do i let it play until the offense complains? if they do , what is the penalty? is that an illegal substitution and the 10th player is restricted to dugout for the game? how can i be illegal substitution if he never really sub'd for a player?

I do a lot of little league (13 and under) and these crazy situations come up all the time, and i was just wondering the correct course of action.