You might think that all the attention to categorization and discrete definition of schizophreniform illnesses in the DSM would make for easy identification of new instances of each illnesses, but this is not always the case. There are many potential causes of psychosis, and just knowing that someone is psychotic doesn't help you to know why they are that way. Information on the proper cause of symptoms is, of course, important if accurate diagnosis is to be made. Accordingly, doctors will rely on logical thinking and diagnostic rules that help them to differentiate one condition from another when faced with a psychosis of unclear origin.
The basic rule for diagnosis is that physical and temporary conditions that could create a psychosis must be ruled out before it is assumed that there is a chronic mental illness present. Substance abuse, generally involving stimulant drugs such as cocaine or methamphamine, whether prescribed or purchased on the street, or the interaction of multiple medications can cause psychosis, as can various medical conditions. Doctors will thus typically refrain from making permanent diagnoses until after patients have been medically examined, and have been been under observation for a long enough period of time so that any intoxications can clear. They will also generally attempt to access prior medical records that might explain psychotic symptoms, and possibly order tests looking for similar evidence. Special care must be taken to distinguish schizophrenia from pre-existing conditions, including other mental disorders. For example, the manic symptoms characteristic of bipolar disorder can be easily mistaken for paranoid type schizophrenia. Only when other causes of psychosis have been excluded can a diagnosis of schizophrenia be pursued.
In some cases, diagnosis is particularly tricky because multiple causes converge to create an ambiguous result. A person with a hidden diathesis for schizophrenia may get involved with stimulant drug abuse, for instance, and show up at the hospital in a fully psychotic state. In such a case, drugs may have caused the psychosis to begin, but they would not necessarily be responsible for sustaining it. In triggering an initial psychosis, they might also serve as a stressor strong enough to trigger the latent mental illness which would then take over, and fuel further chronic psychotic symptoms. While originally, a diagnosis of substance induced psychosis would be appropriate, over time the diagnosis would need to shift to something more akin to schizophrenia. Only time and a documented history of treatment can point towards an accurate diagnosis.