How can you tell if you have schizophrenia or dissociative identity disorder - split personality disorder?

Find a professional. Both mental disorders you mention are significant clinical disorders that should only be diagnosed by a trained mental health professional who has experience with one or both of these. Both are entirely different disorders. "split personality" is sometimes used by laypersons to describe certain phenomena, but there is no clinical disorder that has this name. In did, a personality may split.
Evaluation. Schizophrenia and dissociative identity disorder (did) are two completely different conditions. Even though schizophrenia means split mind-that really isn't what it is. Schizophrenia is a brain disease affecting 1% of the population and is characterized by psychosis which means hallucinations and/ or delusions. Did, which used to be called multiple personality disorder is very rare.