Being the parent of two teens and stepparent to two, I found these ‘typical signs’ most helpful and have stored them in the recesses of my mind (and here on SuperNannyRules) for future reference.  I’ve already recognized a couple of these tell-tale signs in two of our children and am in the process of taking proper steps to end this before it spins out of control.
Signs of a troubled teen:
Your child becomes more secretive, and it seems like more than a desire for greater privacy