How can you tell them apart ( military brass vs commercial brass ) once they have been fired and all sitting in a bucket together
I don't know that there is a hard and fast set of rules.
I'd start by looking on the bottom of the case.
If you see a colored ring around the primer, it's probably military. Sometimes, the stamp will say .223 or 5.56. Sometimes you can tell by the manufacturer - that is two or three letters on the head stamp. LC is Lake City - that's probably military brass. I have some that are AD which was military made in the Middle East.
I'm sure there are folks who know more about this than I do. Your notion of starting a new thread on it is a good idea.