That's rule number one on the shooting range, It's not quite the same in film or on stage.
In those cases, actors have to trust their prop master or armorer.
Those are the people specifically hired to make sure the gun or the bullets are fake.
Baldwin was handed a gun, and specifically told that it was cold. The person handing it over even called out for the entire set that it was a cold weapon. The director then immediately called places. Because that's how it works.
But the gun was not cold.
Now, the person whose job it was to maintain those weapons was incompetent. Baldwin didn't hire her, he didn't hire anyone. He was one of 10 producers and mostly handled fundraising and script changes.
But he made fun of Trump a few times, and was involved in a gun death in a Trump friendly area. In California the armorer would be facing these charges, and would have faced them as soon as the initial investigation was over, not several years later.