People with schizophrenia usually aren't aware that they have it until a doctor or counselor diagnoses them. In fact, chances are they won't even realize that something is seriously wrong. If they do happen to notice symptoms, like not being able to think straight, they might chalk it up to things like stress or being tired.
If you're concerned that you or someone you know is showing signs of schizophrenia, talk to a doctor or counselor.