The live round was loaded into a Starline Brass casing. It had the Starline Brass logo on it.
So a complete novice would look at it and the other Starline Brass logos on the dummy rounds and say, they all match, so they must be dummy rounds, just like all the other dummy rounds on set, because until Baldwin pulled that trigger, he and everyone else on set would have said that there were zero live rounds on set.
Baldwin wasn't trained to tell the difference between a live round and a dummy round. The armorer was (saposed to be) trained to do that.
As for you claiming to be able to tell at a glance, that's also a lie.
The only way to tell is to hold the round up and shake it. A dummy round has a BB in place of the powder. It will make a rattling noise when shaken.
Dummy rounds for movie sets will sometimes even come with a fake primer, because they're props and meant to look real.
The way you tell is by looking for the logo, and shaking them. That's it.
The Set was cold, i.e. there were not supposed to be any live rounds at all. Baldwin was handed his prop, and told it was a cold gun, This would have felt like a formality, only done to keep up the practice. Because there were no live rounds, and the prop was not loaded with blanks. It was loaded with dummies.
Except someone on a previous film had reloaded some dummy rounds with live ammo, and some of those rounds made it back to the prop company and were re-issued to the Rust set.