The key to telling the difference, he said, is through discernment in prayer on the part of the exorcist and the possessed – and in the potentially possessed person's reaction to the exorcist himself and the prayers being said.
The exorcist will typically say “(a) prolonged prayer to the point where if the Adversary is present, there's a reaction,” he said.
“A possessed person has various general attitudes towards an exorcist, who is seen by the Adversary as an enemy ready to fight him.”
Fr. de Meo described the unsettling reaction that a possessed person usually has, detailing a common response to the exorcist's prayer.
“There's no lack of frightening facial expressions, threatening words or gestures and other things,” he said, “but especially blasphemies against God and Our Lady.”
The Catechism of the Catholic Church emphasizes the importance of distinguishing between demonic activity and mental illness. From paragraph 1673: “Exorcism is directed at the expulsion of demons or to the liberation from demonic possession through the spiritual authority which Jesus entrusted to his Church. Illness, especially psychological illness, is a very different matter; treating this is the concern of medical science. Therefore, before an exorcism is performed, it is important to ascertain that one is dealing with the presence of the Evil One, and not an illness.”