Mental disorder/illness Introduction Diagnosis To determine a diagnosis and check for related complications, you may have: A physical exam. Your doctor will try to rule out physical problems that could cause your symptoms. Lab tests. These may include, for example, a check of your thyroid function or a screening for alcohol and drugs. A psychological evaluation. A doctor or mental health provider talks to you about your symptoms, thoughts, feelings and behavior patterns. You may be asked to fill out a questionnaire to help answer these questions. Determining which mental illness, you have. The defining symptoms to determine which mental illness you have are detailed in the Diagnostic and Statistical Manual of Mental Disorders (DSM-5), published by the American Psychiatric Association. This is used by mental health providers to diagnose mental conditions and by insurance companies to reimburse for treatment. Another method is, Depression Anxiety Stress Scale (DASS