How does one know if one is experiencing a depression?

Loss. Loss of interest in things you use to enjoy, waking up for no reason in the middle of the night, sadness in excess for no reason, mental acuity changes, among others.
Depression symptoms. Neurovegetative signs & symptoms are indicative of depression. These include; lack of energy, lack of interest in usual daily activities, sleep pattern changes, appetite changes, feelings of hopelessness & helplessness & guilt, poor concentration, low self-esteem, negative thoughts.