That's not an easy question. A lot of the behavioral stuff such as generally withdrawing and becoming less communicative can also just indicate being a teenager. Grades in school can suffer. That would be pretty common - but not proof positive of anything.
I mean, there are the obvious things such as smelling like dope all the time - but even then, how do you tell it is not just recreational use?
Same with alcohol stuff. I don't have to describe the symptoms of being drunk - but if it happens, how do you know if it is normal coming-of-age stuff or a problem?
Obviously the frequency of the occurrences is important. If it is happening while driving --> obviously not good.
But then even if it gets to the point where strong suspicion becomes confrontation, you usually just get a bunch of denials that you can't really know are lies or typical denial of actual addiction.
Of course different addictions will have different signs. Some addictions come with syringes or various paraphernalia that might be found.
Sorry for the vague answer so far. Problem is, sometimes you as a parent just can't win. My parents had an impossible situation with me. It sucks.
Leave this with me for a bit and I'll see if I can think of something to say that someone could better sink their teeth into.