It may sound way too simple, but if you want to know what your teen is up to take a ride in his/her car.  Of course not all teens have their own car, but often there is a car which is driven most by the teen and it may actually be his/her own car.  With this in mind, each person in the family often has the car that he/she drives most.  So when it's the weekend, your teen knows errand running will be in mom's car.  Shake it up this weekend and say, 'Hey why don't you drive AND let's take your car.' 

It is important to be random and occassional with your teen to remind him/her that you are a smart parent.  It's a simple thought, but when a teen believes that you are not bright he/she will attempt to get away with more.  It's kind of like, 'Well I might as well becasue my parents will never know anyway.'

This is why being a passenger in your teen's car is key.  Realistically if your teen is participating in drugs, or alcohol, it has happened in their car.  If your teen is smoking cigarettes, it has happened in that car.  If these behaviors are occurring, you will know.  It is important to be involved with your child, because they crave structure and advice.  (They don't actually know this, but I'm sure you do!)