I've told them many times that if I ever suspected them of drug use, I'd have them tested pronto. I've told them that that doesn't mean that I believe they're going to do drugs - only that I know the temptation will exist and that many will try to entice them to do so, and that many kids fall to that temptation and don't get help when they need it. I love my kids, and they know it. If I suspect them of drug abuse, I will test them and help them in every way I know how.
Have you told them they will be tested, by you, before they can get their license?