How do you differentiate between the negative symptoms of schizophrenia and depression?

Onset and sadness. Negative symptoms of schizophrenia have significant overlap with depression, making diagnosis challenging. Differentiating these two requires review of history, in particular, how and when the symptoms started. Negative symptoms are more stable over time. Depression symptoms tend to fluctuate, often independent of other schizophrenia symptoms. Sadness, crying are also suggestive of depression.
Follow over time. Schizophrenia is typically diagnosed only when there is an absence of a predominant abnormal mood, since there is so much overlap between depression with psychotic features or bipolar disorder with psychosis and schizophrenia. There are non-overlapping symptoms. The key to differentiating between these two disorders is to track the course of the symptoms over time, both in the past and future.