Talk About It
Asking a 10-year-old to talk to you about bullying is hard enough. Asking a teenager can be downright ghastly and seemingly pointless. But talking to your teen is a strategy that can't be skipped, and it may prove more fruitful than you think (even if your teen brushes you off).
You can be direct ("Is someone giving you trouble at school?") or indirect ("I've noticed you seem anxious when you leave for school -- is something wrong?"). As long as you say something and say it without judgment, accusation or disappointment, you may find your teen was just waiting to say something about it.
Or, you may get the standard "I'm fine" response. In the latter case, don't push too hard. The main point is to show a possible victim of bullying that he or she is not, in fact, alone; that someone sees what is happening, cares what is happening and wants to help.
In the event that your teen wants to talk, the next step is to listen -- and then, if possible, offer some helpful, not harmful, advice about coping with the problem …