Everything's a sign of depression... And that's the problem really, because it can so easily be mistake for something like prolonged sadness, anxiety, or even anger management issues. With young people it's especially difficult because everything makes young people feel upset.
So, for starters, I agree that therapy is a waste of money if you truly have depression because there may be no good cause for your depression. You could just suddenly recall that time you said something stupid a decade ago and feel miserable the rest of the day, or anticipate an argument that will probably never happen but still be reduced to tears over it. Depression can also cause physical discomfort or even pain. So it's more of a mind over matter issue, where you have to recognize the signs of depression coming on and talk yourself out of it. But, of course, a lot of people do well with proper medications.