Possesion only attack person who was have empty mind in that time and have weak mentally with weak soul.
That will open gate in human body for spirit or demons enter and posses that person.
In this case, there a few way for recognize if that person was posses condition or not...
1. Look deep into their eye, sometimes when that person is your friend, you will recognize that eyes is your friend eyes or not.
2. Someone who posses with spirit or demon, they will have more energy than normal human or can do something weird which normal human can't do it and can talk other language they never learn before.
3. They sudenly scream without any reason and act weird (this ones the sign of beginning from demon or spirit posses them).
If you see this and in that situation, don't be panic.. Try hold the mother of finger feet from person who posses and read holy pray from ur believe from religion for rid negative spirit... If the person who posses try fight, ask your friends hold that person body and keep hold the mother of finger feel from that person who posses while u keep read pray.. (sometimes u need try harder squish that person mother finger feet..).
For posses there are 2 ways..
1. Call the spirit for posses you with pray or spell
2. Posses because spirit or demon wants enter your body.