If and when God speaks to you, you will hear it and know it was his voice. That is how I believe, I think he has spoken and still speaks to people. Some of those fearing they will be put in an insane asylum won't tell that God has spoke to them. Others don't recognize it, and some refuse to think that God may have spoke to them.

To keep this on topic, John Brown thought God was telling him to do what he did, but Satan talks to people too,,,,