Demonic possession in the strict sense of the word is rare, and involves drastic symptoms: the voice changes, the personality becomes completely different, fits and convulsions are common, and so on. It doesn't seem to be contagious from person to person. Demonic obsession, in which a negative entity controls or influences someone's thoughts and feelings without actually evicting them from their body and taking it over, is more common, but here again it's not usually contagious. The things to watch for are thoughts, speech, and behavior that are really bizarre; demons don't understand our world, and their logic has been described as "1 + 1 = petunia." If you've got an actual case of demonic influence, the behavior of the people being influenced just plain makes no sense.