Can you tell me how could a psychiatrist tell the difference between a person who has schizophrenia and a person experiencing psychosis?

Careful history: There are many kinds of illnesses in which a person can experience psychosis. Schizophrenia is one such illness - probably one of the most severe illness of this kind. Careful observation and interviewing of the patient and of patient's loved ones is usually what helps the psychiatrist to make a particular diagnosis. Hope this helps.

History: schizophrenia is a chronic condition lasting many months to years. A person who has psychosis for only a short period of time, for example due to using recreational drugs, may not have schizophrenia.