Realistically, I don't think you're going to "check" it. I mean, is he required to remove each bullet and inspect them himself?
Either way, the rule is to treat every weapon as if it were loaded. He failed at that when he had it pointed it at someone, intentionally or not, and pulled the trigger.
The failure isn't solely Baldwins fault. Ultimately the gun was in his hand, but clearly more was happening (or not happening) on that set that compounded the accident.
If he was handed a gun he thought was empty, that's the fault of who ever prepared the gun and handed it to him. The armorer would tell you, your gun is loaded and live.
You should still check out of habit and not take anyone's word for it. That's just part of the protocol
Apparently it was a rehearsal and they weren't even filming an actual scene...
If a real bullet actually ended up in that gun...