How can u tell if the person u live with is a paranoid schizophrenic?

I truly think my "mate" is paranoid schizophrenic...his sister was diagnosed at the age of 55 yrs old, if you say anything to the effect (even in kidding situations) that he has a problem or issue he turns into someone i do not know and is at the brink point of hitting me screaming in my face he does not have a problem...does this sound like someone with paranoid schizophrenic? Please help.