I'm a teenager and I'm Clinically Depressed. I always knew something was off. I was diagnosed in a Hospital. But, I believe your question is how can you tell. Well, a depressed teen is normally despondent. Look for that. Also, always ask yourself WHY they're doing such things. Why are they acting out? To get attention? I know this is all sounds stupid, but really it happens. I don't really know anything else to say but if you have questions about your teens, I will happily answer them (:.