There are standardized depression screening questionnaires such as PHQ-9. Those are at least somewhat subjective, but they are widely used in large scale double-blind studies of mental health treatments. If a large fraction of subjects report a significant improvement in symptoms relative to the control group then we can have some scientific confidence in the treatment.
Brain chemistry such as neurotransmitter levels is clearly a factor in some mental health conditions. There is no serious dispute of this fact. But it's also clearly not the only factor.
http://www.cqaimh.org/pdf/tool_phq9.pdf
Brain chemistry such as neurotransmitter levels is clearly a factor in some mental health conditions. There is no serious dispute of this fact. But it's also clearly not the only factor.