The difference between a blank round and a live round is easy to see. Even then, it is not wise to point a gun loaded with blank rounds and fire at someone. Although it does happen if it is deemed necessary for the movie, but pointing the gun at the production crew should not have happened.
However, the difference between a live round and a dummy round would be very difficult to tell. This is the point of dummy rounds. They are used for close up filming of the ammo but will not fire when loaded into a gun and the trigger pulled. I suspect this is what went wrong. A live round got mixed up with dummy rounds. The armourer believed that the gun contained a dummy round. The assistant producer shouted “cold gun” when handing the weapon to Alec Baldwin. Unfortunately and tragically, the gun was loaded with a live round.