First, let’s review the symptoms.
According to the National Institute of Mental Health, a depressed teen will experience the same symptoms of depression as adults (profound feelings of unhappiness, loss of interest in pleasurable activities, relentless fatigue, etc.), but those symptoms manifest themselves in ways that can be difficult to distinguish from normal teenage behavior. Depression in teens feels the same but it looks totally different than what you’d expect in adults.
So it’s vitally important that you have good communication with your teen about mental health.
Talk to them about what’s normal (feeling sad occasionally) and what isn’t (feeling sad constantly). Ask them if they’re having any problems with bullying, social rejection, or pressure to perform well in academics or extracurricular activities. Most importantly, let them know that it can get better.