World Health Organization stated that Schizophrenia is a severe mental disorder, characterized by profound disruptions in thinking, affecting language, perception, and the sense of self.
A classic and interesting test developed for schizophrenia is the Feldman-Drasgow Visual Verbal Test based on the assumption that patients with schizophrenia exhibit disordered thought that disturbs the capacity for abstract concept formation. The VVT was shown to be substantially effected by intensive psychotherapy in the Michigan State Project (Karon & VandenBos), and changes corresponded to improvements in comprehensive clinical status ratings. Very interesting test, and theoretically sophisticated re core symptomatology of schizophrenia.
Besides 'tests' and assessments to determine the diagnosis of schizophrenia I would advocate getting to know the person and developing an understanding of their experiences. Therefore using tools such as the Belief About Voices Questionnaire (BAVQ [Chadwick, Lees & Birchwood 2000]) and/or the PsyRats [Haddock et al 1999] tools for hallucinations and delusional thinking may prove of great value and also allow the person to be listened to as opposed to facing what can sometimes feel like a barrage of questions.