Demons cannot influence the mind, Only Satan has that power. Demons roam the earth for one purpose, and that is to possess any human body they can get in to to torment . One of Satan's weapons to keep as many in darkness as possible. They walk through dry places in the earth looking for a home,the human body, to dwell.There are different levels and degrees that demon possessed people have. from the smallest which is very hard to detect if you don't know what to look for. to the most obvious. one that is hard to detect is a person that tells you they don't want to hear the gospel of Jesus Christ ,having great anger that could lead to bodily harm.