Here is a list of problems that can indicate that your child is up to something. it could be a lot of different problems, so just be sure to keep an open mind. Children, especially adolescents, give off many of these signals when they are distressed in other ways, too. You'll need to sort out with your child exactly what the problem is, and try to identify any substance abuse, perhaps with the help of a family counselor or child psychologist. Keep in mind that even if there is no drug abuse going on yet, these other problems need to be addressed.

1. Increased truancy and tardiness to classes

2. A drop in grades

3. Behavior problems in school

4. Losing interest in activities the child once enjoyed

5. Dropping old friends and a sudden influence of a new crowd

6. Personality changes -- a child who was once outgoing is now withdrawn, or someone who is usually relaxed is now fidgety much of the time

7. Sudden mood changes -- euphoria followed by tenseness or edginess; excessive suspiciousness or paranoia

8. Appearing listless or hung over

9. Increased forgetfulness

10. Increased secretiveness

11. Withdrawal from the family

12. More combativeness at home

13. Red eye

14. Weight loss

15. Not sleeping well

16. Fatigue or hyperactivity

My wife found these somewhere on the internet I dont know the site tough.

I have to say having a teenage child is the hardest thing I have ever done. Its harder than staying in shape, harder than collage, and it does not seem to get much better. Recently my oldest has been a real pain, all he ever seems to do is go out with his friends, and never tell me what he really doing. I suspect he may be smoking pot, as the other night he came home and I could smell it like he was made of the stuff! I just don't know what to do anymore. My wife says she got this ebook about "talking to teens" I have not taking a look at it yet, but my wife says its a great book. I have been to busy to look at it, but I have to admit the relationship between my son and wife has gotten better. I guess the other night when he came home smelling like pot my wife talked to him the next morning and they had a good talk about were he thinks he is going in life.

A way to monitor kids spending time on the internet, I used to use it on my kids, but now there are too old for that now.
PC Spy

This is some website my wife found one day while looking around on creigslist for a babysitter.
Find A Baby Sitter

And of course here is the link to the talking to teens ebook that my wife bought. I have not looked at it myself, but my wife swears by it.
Talk To Your Teenager