answer:I don’t think it’s easy. You have to do it by process of deduction, or perhaps experimentation. Or you get verification from others as to what they hear. Otherwise, all on your own, without seeking independent verification, you can not tell if a sound comes from the environment, or if it is something generated inside your brain. This is a problem not just with sound, but with sights as well. It is difficult to determine what is a hallucination sometimes. The important thing is remain skeptical, and be open to the idea that your brain is making it up. It isn’t real.