TL;DR get the origin to send you an email and look at the email headers. This won't work for lots of sites. I'd expect it probably also doesn't work sites that

TL;DR get the origin to send you an email and look at the email headers. This won't work for lots of sites. I'd expect it probably also doesn't work sites that use most mail API providers, but maybe some of them have SMTP interfaces and preserve the headers.