An excerpt from "Got Questions.org" entitled, "How do we distinguish a psychological disorder from demon possession?" states: "We know from the Bible that demons can and do possess those who do not belong to Christ, and Scripture gives some examples of people being possessed by demons. From these descriptions, we can find some symptoms of demonic influence as well as gain insights as to how a demon possesses someone. In some of these passages, the demon possession causes physical ailments (inability to speak, epileptic symptoms, blindness, etc. [Matthew 9:32-33, Mark 9:17-18]); in other cases the demon causes the individual to do evil (Judas is the main example); in Acts 16:16-18, the spirit apparently gave a slave girl some ability to know things beyond her own learning (a spirit of divination); in the case of the demoniac of the Gadarenes who was possessed by a multitude of demons, he had superhuman strength, cut himself, went around naked, and lived among the tombstones (Mark 5:1-17). King Saul, after rebelling against the LORD, was troubled by an evil spirit (1 Samuel 16:14-15; 18:10-11; 19:9-10) with the apparent effect of a melancholy mood and an increased desire and readiness to kill David (God’s next anointed king of Israel).
Second, we should be greatly encouraged to know that it is impossible for a Christian to be possessed by a demon. The believer is indwelt by the Holy Spirit of God who comes to reside in our hearts when we give our lives to Christ (2 Corinthians 1:22). The description of a possessing demon as “evil” means “unclean” in the Greek (Mark 5:2), therefore making it impossible for the Holy Spirit to share His dwelling place with such a creature. For those who do not have the Holy Spirit, however, no amount of “cleaning up their lives” will prevent a demon from possessing or influencing them." This article may be viewed at: How do we distinguish a psychological disorder from demon possession?/gotquestions.org
Reference: James 3:16