When someone reports a possible case of possession to the Church, an investigation begins. Father Benedict Groeschel, a Franciscan priest who holds a Ph.D. in psychology from Columbia University, was the man the Archdiocese of New York called on to investigate cases of apparent possession that landed on its desk in the '70s and '80s. In "American Exorcism," he describes his experience this way (Cuneo 22):
... when cases were referred to me I usually sought the help of a laywoman in the archdiocese who possessed a gift for discerning spirits. In her view, and also mine, none of the people I brought to her were victims of possession; none of them, in other words, were in need of formal exorcism. But that doesn't mean they weren't being afflicted or oppressed in various ways by demonic presences. Demonic oppression is much less serious than full-scale possession, and it can usually be dealt with by what we refer to as a simple prayer of deliverance.
A typical investigation is essentially a process of elimination: Does the subject exhibit the telltale signs of demonic possession? Is there any other way to explain the subject's behavior besides demonic possession?
Often, the priest will consult a psychiatrist in his investigation in order to determine whether the "possessed" person's symptoms can be fully explained by mental illness. According to Michael Cuneo's "American Exorcism," there are about a dozen psychiatrists in the United States who evaluate potentially possessed subjects for the Catholic Church. The subject will also undergo a medical examination to find out if the symptoms can be attributed to a physical disorder or illness. The priest may consult a Church-approved expert on the paranormal for additional input. Another possibility the investigator must consider is plain old fraud.
If the priest is convinced of the validity of the possession and that an exorcism is the appropriate way to help this person, he will report back to his supervisor (in most cases, the diocesan bishop) that an exorcism is in order. The Church may then decide to sanction an official exorcism and appoint an exorcist to the case.