Check your child's cell phone records for any number that appears to call or has been called more frequently than others. Pay special attention to the time and frequency of those calls.
Monitor your teenager's internet history - chats, emails, etc. Find out what he/she was talking about to others.
Let your teenager know that he/she can come to you no matter what. That way, he/she will be more willing to tell you what is really going on.
Check for bruises and monitor how your child claims he/she got them. Observe if your child is covering him/herself up more than usual.