Well, first of, when someone's being possessed, they don't act like themself.
Second, the moment someone gets possessed, the room suddenly gets to a freezing cold sensation. Even if you're under the bright warm sun.
And finally, When a possessed person speaks, they'll always, and I mean ALWAYS, speak in multiple voices. One is their normal voice, while the demon is making them speak, so it turns into a demonic, deep voice.
That, is my proof that demons are real. Need more proof? Don't worry, I got more.