They diagnose you based on what you have told them so it can be easy to overlook because psychosis and schizophrenia usually involve a lack of insight into the condition. Like ILADVOCATE said, psychosis can be caused by any number of conditions however.
Psychosis can be caused by any number of disorders but in order for a person to be diagnosed with schizophrenia they have to have experienced psychosis, including auditory (and other) hallucinations ("hearing voices") and thought disruptions of some kind such as delusions.