I've learned about the voices that come from the enemies of God and mankind. Some of them will claim to be God. You know that it isn't because they are speaking to those who are unbelievers.
The spirits lie.
Those unholy spirits often torment and try to stress the person.
They tempt those people to sin and to harm themselves and others.
Some who speak to New Agers and eastern gurus, will claim to be an alien, higher consciousness, dead relative, famous person,or God; these are all devils/ demons.
Many schizophrenics suffer from demon possession. I've been studying this subject recently.
I have never heard one through voices in my head or anything like that because I'm a saved believer in Christ. However, I have had several speak audibly through others. One tried to murder me without warning. I was scared until the Lord brought to my mind loud and clear as He applied it specifically to this creature. The Lord told me,
"Greater is He Who is in you than he who is in the world."
Immediately the fear left and I knew for certain what I faced was my mortal enemy. I looked into the face of a demon as manifesting through it's chosen host.
The Lord was in His temple and there was no reason to fear....only for His power to cast the demon out. He had the victory.