How can you tell the difference between a hallucination and real voices

If you cant verify something with a source outside of yourself don't depend on it. Also use as much logic as you can. Seeing or hearing spiders talking to you doesn't reflect reality and you can trust that those are in your head. It gets more confusing when it is the voices of people. But are those people anywhere nearby to even speak? It's hard but there are a few things like this you can do. And always educate yourself on your condition. Knowledge is power.