Depression can be difficult to diagnose because many of its symptoms, such as fatigue, mimic those of physical illness. A good diagnostic evaluation also will include a complete history of the symptoms, i.e., when they started, how long they have lasted, how severe they are, whether the patient had them before and, if so, whether they were treated and what treatment they received.

Alcohol and drug abuse should be asked about.

Further, a history should include questions about whether other family members have had a depressive illness. Last, a diagnostic evaluation will include a mental status examination to determine the speech and memory pattern.

Diagnostic methods may include:

• Physical examination - to check for any underlying physical illnesses. 
• Medical history - for example, asking about prior use of various drugs (such as high blood pressure medications, steroids and alcohol), which can cause depression as a side effect.
• Detailed questioning - for example, asking questions about the severity and duration of the depression, any triggering life events and so on. 
• Psychological tests - to further help identify the type of depression and its contributing factors.