> how would you even know?

The obvious way is by searching the training data for close matches. LLMs need to do that and warn you about it. Of course the problem is they all trained on pirated books and then deleted them...

But either way it's kind of a "your problem" thing. You can't really just say "I invented this great tool and it sometimes lets me violate copyright without realising. You don't mind do you, copyright holders?"