How do doctors and psychiatrist determine that mental illnesses such as schizophrenia are purely psychiatric and not organic or autoimmune related? what tests are done?

By UPTODATE: "The diagnosis of schizophrenia requires the presence of “characteristic symptoms” of the disorder (delusions, hallucinations, disorganized speech or behavior, and/or negative symptoms) coupled with social and/or occupational dysfunction for at least six months in the ABSENCE OF ANOTHER DIAGNOSIS that would better account for the presentation". So, tests may need to be done at times.