You can read the latest about the sweet old Father Amorth here, but I highly recommend this hilarious 2001 FreeRepublic interview in which he is asked how one might recognize someone possessed by the devil
By their aversion to the sacrament and all things sacred. If blessed they become furious. If confronted with the crucifix, they are subdued. We can sort out the phoney ones. We look into their eyes. As part of the exorcism, at specific times during the prayers, holding two fingers on the patient's eyes we raise the eyelids. Almost always, in cases of evil presence, the eyes look completely white. Even with the help of both hands, we can barely discern whether the pupils are towards the top or the bottom of the eye. If the pupils are looking up, the demons in possession are scorpions. If looking down, they are serpents.
I suppose this is an important distinction because one wants to have the appropriate venom antidote on hand.
Things like this tend to erode the believability of the Vatican. (Ha. Snort.)