Mother's gut feeling is very correct. Kids will try their best not to show any signs of drug use. Once they get caught few times being stoned, they will start to carry "Visine", cologne and other stuff to hide. When this happens, 1. They do not have plan to stop using whatever they are using. 2. They want to hide the visual facts that can be seen. 3. They will deny when confronted. With my experience, these behavior will not stop.
If the child is an adult, you cannot force him to take the drug test or anything as they will say "I am an adult and can do whatever I want. This is none of your business" with attitude.
They will also say 1. You do not know what you are talking about. Pot is legal, natural and doesn't do any harm to humans. (Well, you have to be careful as it is in fact legal for medical use and some states are legal for recreational use) If I look at so called pot head friends that I saw in high school and college, majority of them can't stop and became not that productive in life. It is a circumstantial thing and kids will challenge. Yes. There are people out there smoke a lot and being successful. But we are talking about very small percentage in my humble opinion. It doesn't mean that your child will be the same case. If it causes problems, it is not working for the kid. If your child gets into advanced level of smoking such as dabbing, which uses concentrated forms of pot and oil, then the situation becomes more severe and serious.
For hard drugs, I can only imagine... This would be really tough on parents. We can't force them to stop. We will have to find a way for them to realize by themselves that doing whatever they are doing is not good for themselves and hurting other family members. Making them realizing part is the most important I think.