Where you watching CSI MIAMI re runs last week? It is very difficult for a professional to distinguish personality disorders let alone a layman. There are actually very few true cases of it on record. The problem might be drug related,or teens until they get past age 25 approximately, are prone to schizophrenia.
They will seem different. I myself have DID. and you can tell when one of my others has taken over. multiple personalities are not subtle. You can tell that the person isn't the same. You should also go to a psychiatrist to get diagnosed. That's what I did and found out I have DID.
Well, for one its very rare. Mood swings that present in other mental illnesses like Bipolar Disorder and Schizophrenia can often SEEM like split personality. In fact im not sure just anyone could tell since such a thing is easy to disguise. A specific set of diagnostic criteria have to be present and this usually must be determined by psych testing and counseling sessions and observation. It certainly isnt like they show on TV.