Be persistent and learn what it is that they are trying to cover up. Drugs are always a possibility. Dramatic mode swings. This is often identified as normal teenage behavior - and it may be, but it can also be a sign of drug abuse. Excessive money requirements or money disappearing.
Drugs cost money - lots of money.
If your child keeps coming to you needing money, or if money keeps disappearing from your purse or wallet, you need to have a serious talk with your child. Talking too slow or too fast. Pot tends to put a person in a stupor and affects a person's thought process. If your child is using stimulants, they will speak very fast and act very hyper. Look for dramatic changes in your child's pattern of speech.