How can I tell if my friend is schizophrenic?

It may be difficult. It is difficult to tell sometimes, as many with paranoia are able to "hold it together" around others. Some with schizophrenia and while they are psychotic may talk to themselves or may say some bizarre things which don't really make any sense. It can be difficult to tell whether or not certain folks have schizophrenia.
See a psychiatrist. Schizophrenia is a chronic mental disorder characterized by hallucinations and delusions. There are 3different subtypes, but the most common is paranoid.