The symptoms of depression and criteria are contained in the DSM. They don't involve brain chemistry or functioning (per se), e.g. "Recurrent thoughts of death, recurrent suicidal ideation without a specific plan, or a suicide attempt or a specific plan for committing suicide."

The underlying reasons for those symptoms may involve chemical imbalances, but those aren't established or tested for. They don't take a CSF sample or do an MRI for a diagnosis.