Non-Invasive Drug Testing
If you ever want to know if one of your European-traveling friends smokes pot, just ask what they think of Amsterdam. If you get a response along the lines of, "It's okay, kind of a shithole," then you know they don't smoke. If they respond with unqualified enthusiasm, then you've got yourself a smoker.
Note: exceptions can be found if the person if obsessed with either french fries or Rembrandt.