Failing something like diffing which is the best way you might consider a tool that enables you to print the document contents to another document, either another type of document (epub to pdf for example) or the same type and doing that should result in no metadata at all carrying over. Of course you need the right tool for it and it may result in all kinds of mishaps with shifted text and pages and other nonsense so diffing would be better while this printing to another file solution would be nuclear but likely to foil everything but very advanced methods not likely to be employed to prevent piracy and basically impossible to automate.

I'd open the files up in Calibre or another viewer and see what kind of info it shows. Try to strip that, then open the result in a hex editor and try searching for your registered email address or phone number within the file which is the low hanging fruit obviously as if I were Amazon I'd use an internally known account number in either plaintext or even better encoded in unprintable bytes. All in all I'd try the printing to/converting to another format trick or consulting people more knowledgeable about this about methods and what to do.