That is the question that that clinicians often have to ask. Depression certainly operates along a scale and it is often difficult to quantify either i) if a person is depressed or ii) how depressed they are.
One option to assess patient depression is the PHQ-9. The assessment asks patients nine questions regarding their mental health status. The patient’s depression level can be quantified on a scale from 0-27 and practitioners can use this information to provide a tentative diagnosis.
The PHQ-9, like many assessment, relies on patient self-reports which depend on the communication skills of the patient. Further, depression can be transitive in some cases; thus additional PHQ-9 questionnaires will need to be conducted over time. Still the PHQ-9 can be a useful tool for physicians assessing patient depression.