Well, generally you can't. Now if a person was constantly hearing voices, and didn't otherwise make claims of religious leadership, and under the right conditions, then they would likely be dismissed as mentally ill. Maybe even confined for it. But when people who are not otherwise know to be mentally ill start saying god is speaking to them, and they have some message that resonates with at least a few people, then they become prophets. Hell, most religious groups probably started off with crazies. Those that didn't start with fraudsters.