Dummy rounds are easy to check. Point the gun at the ground, in a safe direction, cock and pull the trigger. Six times.
Movie dummy rounds usually have holes drilled in them.
Dummy rounds have a telltale. Holes drilled in the case, missing, drilled or dimpled primers, a pellet inside that rattles when you shake it, etc. The armorer knew how to identify HER dummy rounds, and the crew should have been trained. No one checked the gun. No one. The set was a circus and Baldwin was the head clown.