You can do two things to gain access to passwords from the system, this will disclose other stuff as well - such as what email was used on the apple site, etc.. Disclosing the apple id obviously.
a) key chain. Open it, unlock it, browse through it, take note of the details in there.
b) chrome password manager. open chrome, go to settings, find manage passwords, and unlock to 'show password' , and take note.
The combo of these two will be enough to see what sites have been visited, or at least have a login to via a submitted form, and which apps etc that use keychain (like iTunes probably) will have some details.
You can also indeed try to open iTunes and see if it lists the email, but fails the login if the pass is unknown. But if you have the email, you can restore apple-id through apple's site.
That said, modern versions of Apple's macOS (osx) are free for download now. The older ones were for sale for like 15 bucks. Maybe creating a new apple id and saving the headache of figuring things out over just getting a new apple id might be an option to consider