Self-reporting is probably ideal, probably with a camera recording everything so I can judge my answers and behavior and compare it with my memory. But we don't really have a great handle on how much of our memory of conscious experience matches our real-time experience or even if the memory-formation process might "be" consciousness, so I admit there are blind spots in studying consciousness rigorously.

I don't buy the philosophical zombie argument simply because consciousness does alter behavior. I wouldn't participate in this conversation the same way if I didn't experience consciousness. It would be more like vivid imagination (as apposed to moderate aphantasia) where I find it curious but don't have it. As in the novel, unconscious beings probably behave noticeably different.

There are, apparently, some people who have a very reduced sense of consciousness. I know I have done and said things when I'm not (memorably) conscious, for example when half asleep or coming out of anesthesia, and my behavior has been altered according to witnesses. I wasn't quite "myself". I can also hyper-focus and reduce conscious awareness of my surroundings and of my own body and mind, but that still feels like I have an internal awareness and memory of the experience. I am fairly certain I would be able to tell if that is switched off for a time.