How can they prove who did what to whom?
LE don't usually use same weapons as general public but we can know for certain as LE weapons are registered to each person on the force and others won't be the same.
There is a huge cone of silence surrounding the circumstances that led up to this massive battle, like, we do not even know (officially) the biker gangs that were involved, or what the contention was about. And it most likely was NOT parking space.
What this was not, was white cops shooting down unarmed black men who found themselves in a confrontation they did not want. Totally wrong narrative.
IF they want to, ballistics, wound angles and blood spatter would indicate what shots came from what direction. If the police were there, there should have been at least one recorder going, unless they wanted to start a war between outlaw biker factions. If the latter is the case, the police may have done most of the shooting and will pass the 'credit' on to others.
I read they were still tracing bullets to see who shot whom.