It is written on all 9mm cartridges regardless of who manufactured the ammo. It would be hard to distinguish it from say 40 S&W ammo from across the room, but my guess is the reporter just read what was on the cartridges.
"9mm Luger" is about as specific as saying there was a 12oz soda can on his table. The details about a specific cartridge are only written on the box, but can be determined from a casing and a photo book. If he said he has "124 Gr Remmington Golden Saber 9mm Luger" ammo, that would actually be specific information.
Looking at a cartridge from a few feet away, anyone who's a gun geek and has decent eyesight can probably tell you that a 9mm cartridge is either that, a .40 S&W, or something very obscure. Also though, almost all handgun cartridges have their caliber stamped on the rim. I just checked to see from how far away I can read the markings on my Federal .40 (I don't own any 9mm, but it should be basically comparable). In good light and at just the correct angle, I can read it from about arm's length. Otherwise I have to hold it close.
"Luger", originally, meant the Luger P08 pistol. You'd probably recognize it as the sidearm you see the Nazis carrying in WWII movies. It was the first pistol to take the cartridge which today most people just call "9mm".
Today, "9mm Luger", "9mm Parabellum", and "9x19mm" are all synonyms for this same cartridge. Usually when "9mm" is said without qualification, this is what it means. However, there are other much rarer 9mm cartridges such as the 9x18mm Makarov, and also "9mm short" and "9x17mm" are rarely-used synonyms for the (common) cartridge usually called .380ACP.
It might seem like a small difference if you're not used to handling pistol cartridges, but if you are, that's like saying it's difficult to tell PHP from Perl.
Edit: Also, there is usually an imprint on the bottom of the cartridge that says "9MM LUGER".
I handle a lot of guns and that is a great analogy. We know that all we have to do is look for a ".php". But how many average people notice that type of stuff?
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.
You're assuming that the author is an average person with respect to gun knowledge. I don't know how much he knows about them, but the story seems to suggest he knows a little, right? Maybe relevant: he was a war correspondent in Iraq[1]. Or maybe he just likes guns?
There's also a much more obvious difference in "fatness" (e.g. width/length). Humans are much better at judging that kind of relative difference quickly and from a distance....
An occasional, recreational shooter aught to be able to recognize 9mm cartridges fairly easily. Here are a couple of lineups comparing 9mm with other pistol cartridges.
I don't think there's "Luger Brand ammunition" as mentioned in the police report, either (unless it's some lame latin american brand I've never heard of)
Rather, it's a brand/mark, but not that of the ammunition manufacturer. The (9mm Luger|9x19mm|9mm Parabellum) is the round used by most 9mm handguns, and is manufactured by thousands of companies worldwide. "Luger" is simply the name of the round's designer and the Luger pistol that introduced it.
9mm luger is easy to identify for someone familiar with handguns. However, 9mm luger is one of, if not THE most popular handgun ammunition in the world, so in my mind, it doesn't even imply circumstantial guilt.
