Unless he somehow saw the box or the imprint at the bottom, it's almost impossible.
There is a small difference in width but you can't really tell if you're further than a foot away.
There is a small difference in width but you can't really tell if you're further than a foot away.
Edit: Also, there is usually an imprint on the bottom of the cartridge that says "9MM LUGER".
You're right that there are imprints on all rounds but seeing and remembering that is like getting a database error and remembering if it was Microsoft or MYSQL...You have to have a lot of experience and a keen memory.
So pretty much impossible.
[1] http://www.wired.com/wired/archive/11.06/battlefield_pr.html
Edit: spelling