How can I be certain that I need tests for schizophrenia?

If you're asking... The fact that you are asking the question suggests that something is happening to you that needs assessment. If you are having "unusual experiences" such as hearing voices or seeing things others don't see--not related to drug use--or otherwise are finding yourself having a harder time with general thinking processes, get an assessment. It may be something else entirely, but you should find out.
Schizophrenia. Firstly, do not diagnose yourself. Get a full psychiatric evaluation and inquire about medication options to control hallucinations and other symptoms. Also get a referral to a reputable psychotherapist who specializes in psychotic disorders. DO IT TODAY.