 There are two ways that you can know that a person actually has a demon, demon possessed, demonized, whichever vocabulary word you want to use. One, when a person has a demon, that demon will always recognize God, the Spirit of God, the Holy Spirit. Every time in the scriptures that person calls out, recognizes who that person is and gives glory to God without fail. Why? Because he's literally the one that created them. Two, the other way that you can know that a person has a demon is because that person is not functional. I've been around people that had some sort of demonic influence and they were not functional. Pulling their skin out, pulling their hair out, cutting themselves. They always live a lifestyle that is damaging and hurtful of themselves or promoting the enemy. People that can otherwise function in society, go to church, read, go to the movies, do these different things, and then have these manifestations on command. That person likely is just simply acting. Or you can find someone that has a stronghold in their life. It's not a demonized person. Whenever that person is loose from that demonic influence, the person returns to a life that is normal where they can function in society.