It is a movie industry standard.
When Brandon Lee was killed by a gunfire accident, Hollywood adopted a set of rules that were mandatory for all usage of firearms on any movie set. I have read this list of rules, and if Baldwin had followed the rules, the accident which occurred would not have happened.
One of the rules *REQUIRES* the actor to verify that the gun is safe. He must physically check it to make certain that it is not loaded with real ammunition. He must declare that it is safe in a loud voice so that the rest of the crew can be aware that the gun is safe.
There are a whole set of rules they are required to follow, and they've been posted regarding this topic in the past.
I have heard talk about these “rules” but have yet to see a link to a full copy. Anyone?