IPad9 Magic Mouse IPad9 how to know when charged?

Ipad9 Magic Mouse. It works intermittently on game(June's Journey) So I've plugged it in to charge. But I can't find # on Magic Mouse for exact instructions. I've plugged it in to charge but can't find where it tells me when it's charged fully.? Also not sure what else to do on iPad 9 other then charging to make it work all the time.