You can see if any stat of any main girl is at maximum (version cap) by checking it in her panel.
Also from the inventory if you have them as a favorite (in easy mode this is not possible).
If you mean the final cap when the game is finished... I haven't decided yet, but it should be level 10 for all stats and all girls, and since there are a lot of girls, it's going to take a while to get there 🤔