Generally, a person who has schizophrenia will display what's called First Rank Symptoms. These are symptoms (such as delusions of control) that were identified by Kurt Schneider who thought that they most characterised schizophrenia. Generally, even having one of these symptoms will be enough to be diagnosed with schizophrenia (at least where I'm from in Australia).

Unfortunately, you can't identify it in the same way as say, Huntington's Disease. When someone has schizophrenia, most of them go through what's call a "prodromal phase", where they start to have cognitive symptoms (such as memory issues) or to develop some signs of delusion-like thoughts. Eventually, what happens is that this develops into a full-blown psychotic episode, requiring hospitalisation. This is where schizophrenia is usually identified.