I'm a person who has heard voices and known it. I can tell you how. It's called 'reality checking.' You can only really do reality checking if you're not having other sever psychosis from what I understand.

For example, once I was in the shower. I had my back to the curtain and an adult male voice whispered in my left ear "Hey!" It was a loud whisper. I screamed and turned around, thinking it was my husband trying to scare me. But he was not in the bathroom, and was in fact on the other side of the house alseep in bed. The only other person in the house was my 5 year old son, also asleep soundly in his room.

So, reality check. Either a) it's a ghost or demon or something super natural, or b) I heard a voice and it's an auditory hallucination. The more logical answer is B, especially since I have a history of auditory hallucinations.