There is no sure-fire way of knowing if your teen is drinking (many do, many experiment with alcohol and the reality is there is no way to watch a teen 24 hours a day, other than house arrest).
       So you may have to face the fact that, much as you dislike it, you can't stop all experimentation, including drinking. But you CAN be aware of possible warning signs that occasional experimentation has turned into addiction.

1. First off, don't make the mistake of saying it is fine to drink a lot. Don't be in denial or pass off slurred speech and DUI arrests as "normal teen behavior." Trust your instincts. Most teens experiment and that can include getting drunk. But if you start to feel alarmed, listen to that inner voice.

2. If your teen gets thrown off  the team for failing drug tests, gets arrested for crashing the car while drunk and then CONTINUES to drink, it could be a sign that alcohol is more important then the rest of his life.

3. Having hangovers every Monday morning. This may sound obvious but I know a few parents who think that binging on weekends is "normal teenage behavior."

4. Hanging out ONLY at homes where there are no adults around  and with kids who have a rep for drinking.

5. Losing friends, getting in a lot of fights with friends, going on to a new group of drinking buddies.

6. Smelling like breath mints or mouthwash all the time.

7. Read books on the topic. I'd suggest a book called From Binge to Blackout which contains a lot of information, as well as the personal experience of one former teen alcoholic.

8. Be especially vigilant if there is a family history of alcoholism. There can be a predisposition to get addicted and what may be normal experimentation for most teens can quickly turn into addiction in susceptable teens.

9. Know the facts. For instance, teens can become full-blown alcohoics in far less time than it would take an adult to do so. It can take years for an adult to be classified as an alcoholic. It can take less than a year for a teen to reach the same stages, with delirium tremors, blackouts, etc. That is a HUGE difference in the timeline. Teens ARE at more risk if they drink heavily.

      Many of the above behaviors, including heavy drinking, could be signs of other problems as well - or could be an attempt to "self-medicate". You may need to consult a professional to get a full picture and recognize if there is more than drinking that is going on -- if there is depression or some other underlying condition. If your teen is a full-blown alcoholic, keep in mind that detoxing at home without medical supervision can be dangerous, even deadly.

Finally, to get even more insight, read the book From Binge to Blackout, which is the story of one teen drinker, told from both his perspective and that of his mother. If you recognize your own kid in those pages, it may be time to get the issue out in the open.