I’ve been seeing in the news recently, quotes like this:
How do they know they were Shi’ite and not Sunni (or other)? Do they base this on their clothing? What they yell as they’re fighting? Interviews with surviving family members? General region of the fighting?
Most likely, because they are fighting in Shia neighborhoods. Many Shia neighborhoods are in turmoil as followers of the Shia cleric Moqtada Sadr are clashing with coalition and Iraqi government forces.
There is a section of Baghdad called Sadr City (named after the current Sadr’s father) which is pretty much a war zone now. It is safe to assume that fighters there and in other Shia enclaves are Shia.