#1: Behaviour.
A teenager who is abusing drugs will not be thinking properly or clearly, and will be responding at a much slower pace than normal.
#2: Eyes.
A drug-abusing teenager has dilated pupils or bloodshot eyes.
#3: Conversation.
Your teenager has most likely already been exposed to drugs at school. Make sure you teach him/her properly about the dangers of different drugs.
#4: Parent, don't dictate.
Simply telling your child what to do or not do will create a communication barrier between the two of you. Guide your teenager and help him/her grow up into a responsible adult.
#5: Grow with your teenager.
Be willing to listen and talk to your teenager. If a teenager feels that he/she cannot talk to his/her parents, they will turn to others, usually friends. Their friends might already be using drugs and thus present a bad influence. Make sure you are always there for your teenager.