According to Exorcismus.org, determining if a person has an evil spirit in them is done by spotting various characteristic changes, such as the development of lewd and obscene thoughts or difficulties praying. An exorcist priest spends his whole life learning to spot these characteristics.
Exorcismus.org says that people who have an evil spirit within them tend to have very aggressive behaviors. These behaviors are amplified when the person is in the presence of religious symbols and gestures. For example, praying in front of someone who is infected by an evil spirit causes the person to become wildly aggressive. People infected with evil spirits also become repulsed by sacramental items, such as blessed water or a saint's relic.