It seems that you may be afraid to approach your independent, strong-willed, academically accomplished daughter on any serious matter, for fear that she would become defensive, angry and possibly leave home. I would say that a relationship that is so poor and tenuous should be the focus of your concerns with her, rather than whether she has tried marijuana. Talking with your daughter about any serious concerns you have about her behavior should not be compromised by your fears of her leaving home.
If you are concerned about her possible drug use, I would ask her open-ended, non-accusatory questions that suggest your knowledge of drug use and your concern for her if she is using drugs. My professional advice would be to get some individual professional counseling from a therapist experienced in family dynamics. I'd also give these books a look: "All Grown Up and No Place to Go," by David Elkind, "When Good Kids Do Bad Things," by Katherine Gordy Levine and 'You and Your Adolescent (Revised Edition)," by Laurence Steinberg.