The manufacturer brand stamped in the side.
The ones I can think of are Cornelius, spartan, Firestone, challenger, I'm sure there's a few I'm forgetting.
I'm not really hung up on maker of the keg as long as it is clean and hold pressure, posts and seals are ok. I'd use it.
I was just recently given a keg with the old racetrack lids. It wouldn't hold a seal so I bought a new lid with pressure release valve. When the new lid came in, new O rings, Still leaks. Looks like the flange that the lid presses against is slightly bent or something...it won't form a seal.