My kid's grades are slipping and he seems distant. How can I determine is he's on drugs?

Testing. To determine if he is on drugs, blood and urine testing can be done. However there are many reasons for getting poor grades and being more distant. I advise you to speak to your child in a nonthreatening manner and see if he can give you insight into what is going on. If you force him to get tested you may cause him to withdraw further from you. A counselor or psychologist may be very beneficial.
Kids on Drugs. Suspect drug use by your child? Get a hair sample first. You may find a drug screening kit online. Best is a urine sample to screen. Ask you son daughter for a urine sample. Your concern for his or her health should override your concern for not offending your child.