I think the answer is no, but I thought I'd ask in case someone knows better.
The application I work on accepts incoming email, I'm trying to find a way to identify a message as having been forwarded (not server-side forwarding, just a regular old "I want to forward this email message"), so I can identify the original sender address (and hopefully then the content).
I do see any indication I can do this via the email headers, leaving me with parsing subject and body for clues, which would be very inconsistent.