I would address any behaviors that are causing problems in the teen's life (ie: falling school grades, not participating in usual healthy activities, mood swings etc...) ....I wouldn't drug test unless the teen agreed.
My children are now healthy, responsible adults. But.
When my son was a teen and going through a rough time, the only time I had him drug tested was with his okay----the school called me as he was acting strangely (I won't go into the prelims....it was odd they called me as he was staying with his dad at the time in his school district---nearby but..)---anyway, when I picked him up he was obviously stoned on something----I asked him what he had taken and he told me a couple of pills he found at his dad's house that he thought were my meds for anxiety (not true, he knew no meds of mine were there or ever accessible to him) --I told him that I wanted to take him to the ER and have him tested to find out what he took, that I would pay out of pocket so that it would not be in his health record etc & he agreed
....long short; he had taken some opiods his dad had confiscated from a group home that he should have destroyed at the home and not brought into the house---we got through the day. This was not the worst time for him
But, generally I focused on behaviors and encouraged all healthy activity (sports, hiking, camping, socializing etc, learning & travel...). Today he is healthy, plays league basketball (for years now) regularly, is a wonderful father, a good worker, and recently went hiking through three countries in South America---my grandson is happy, healthy, and has two great parents. It isn't so much the doing of drugs but the doing of drugs in a way that interferes with productivity that is the problem---and, often, alcohol is the real danger....the real "gateway" drug or drug of choice....