Signs of demonic possession?

Demonic possession is very rare today but growing rapidly due to people dabling in the occult; spirit contact etc. Before someone can be deemed "demonically possessed" one must rule out a number of factors such such as mental illness and/or emotonial. Please see this link for signs of demonic possession.
http://fspp.net/warn%20poss.htm

I believe that demonic possession could be easily mistakened for mental illness, but do some tests. Place water in the victims mouth and cover it with tape. Tell him/her NOT to drink the water. Get some people to hold the person down because things could get out of hand very easily. Do things to make the demon come out. Ask it questions and stuff. If the person could TALK even with WATER and TAPE in his/her mouth, I would consider that possession.

Bring crosses and see if the person freaks out :)