How Can Schizophrenia Be Diagnosed?

Ok so my step daughter says she is seeing dead things and hearing voices, she has been known to be a habitual liar, she just came out of the hospital, where she was in the physic ward for behavior reasons. She is telling her mother she has scitsophric, from what I ve read, she never had an onset being 19 and says she has full blown schizophrenia. How can we tell if she s telling truths or is just acting ,on what she learned form being around people with the true disease,, for attention?