How can we detect symptoms of depression?

Depression. In others or yourself? Let's take ourselves: persistent sad mood lasting over two months, over or under sleeping, appetite changes, poor concentration, irritability, crying spells, lack of energy. When you put it all together, it may be depression - have a talk with your doctor to help decide what your difficulty is.
Withdrawal. Depressed people usually fixate or center themselves on intense negativity (depending on the severity of their depression), and tend to gravitate away from friends and family (who may have far more of a positive attitude). Or else, being family and friends, they may be able to see the depression. Clinical, or non-clinical depression, can eventually lead to the entertainment of sucidal thoughts.