My son's soccer coach said he is noticing signs of teenage depression in my son. What are the symptoms of teenage depression anyway, I feel like a bad mother but I don't even know!

Unless your son's soccer coach is a mental health professional I would suspend judgement until you brought him to someone more qualified to assess his condition. As parents we can actually create problems by expecting the worst and flippant suggestions like the one given by his soccer coach serve no purpose other than creating problems that might not exist.