We cant't know that someone has consciousness by interrogating them (except, perhaps, by asking them their philosophy of consciousness; staunch physicalism is highly suspect, but still only a strong hint to something we can never fully verify). Nor can we by observing their brain, or any other part of any physical system. We grant that Polite Axiom, the fundamental of assumption of consciousness, to our fellow man, but it is never confirmed.

Consciousness is just there. It may "arise" from the complexities of a physical system, but this is, in Mark Bedau's terminology, "strong emergence", something that pops into existence. It is ineffable and not reductively explained by any physical law (as a "weak emergent" phenomenon would be).

Since it is just there, and since it is not weak-emergent from the behavior of neurons, as commonly thought, then we have no guarantee at all that it's "generated" by the brain.

In fact, we don't even know that it's associated with the brain. We assume this axiomatically - the aforementioned Polite Axiom. We assign consciousness to observed behaviors, but we have no certainty at all. Not only no certainty that it's being experienced, but also no certainty that it's "in there". After all, if it's not composed of ordinary physical matter, then it need not be bound by matter's rules.

Perhaps, a brain is just a kind of conduit consciousness uses to express itself.