One of the most difficult tasks for mental health workers is to clinically assess and diagnose mental illnesses - especially when comorbidity (having more than one mental illness) is so common. It usually begins with a psychological assessment through tests, observation, and interviews so the clinician can catalog the symptoms. Then the DSM-5 is consulted to give the diagnosis.
A clinician may focus the assessment in three ways - biological, psychodynamic, and behaviorally.