At times you can't differentiate between voices and your own thoughts they become merged in some mangled mess. You can't tell the difference, the voices become your thoughts, its become so loud and invasive in your mind you can't differentiate.
The voices and paranoia start to consume you, a tiny sliver of your former self tries to weigh reason against insanity but always comes up short. Hence the delusional you, and the real you merge and start fighting the good fight. The delusions can easily take over.
Even after being medicated, you wonder all the time, am I sane, can my thoughts be trusted? Was the sound i just heard, in my head or a true external sound. Its very complex and difficult to understand where you are at in the world at times. For me I have paranoia, about paranoia. I am afraid one day, again, I will lose touch with reality and go back to the bad place I came.
So how do you distinguish thoughts from voices, simply put at times you can't. And even if you could, you couldn't trust your own thoughts... So really I guess you would say in a delusional unmediated state you probably couldn't or would have a hell of a time doing it.