In addition to the header info mentioned in other comments, email providers let you subscribe to email Feed Back Loops. You register for FBLs (man it's a neverending process since there's a never ending pile of new email providers), and when someone marks something as spam, the email provider (gmail/outlook/etc) will send an email to a registered address, you just listen for emails on that address. I wrote a custom handler to do this, as we already had a system in place to process incoming email from users (respond to comment email instead of replying on web form etc). I then unsubscribe the address in question.

Not all providers offer FBLs, in that case my internet provider gets an abuse notice which is forwarded along to me. Not a ton of activity there on a 500k person mailing list, the FBLs are pretty effective.

Also, since we have the email processing system in place, it's easy to detect bounces and auto unsubscribe people. I immediately unsubscribe on a hard bounce, and wait for 5 soft bounces before doing the same thing.