Not to get all weird and spiritual, but I can usually tell the drugs people are into just by looking at their face, including alcohol and nicotine.
I can even tell when someone has been a former drug user and recovered. It's all in the eyes, mouth, forehead, teeth, small wrinkles. There are telltale signs. That's not to say I can tell if someone has tried acid once in College, but the more you experiment, the more the road map of your life is etched into your features.
Unfortunately I don't have much of an opportunity to verify my predictions because it's not really a subject you bring up with too many people, other than close friends.. and I mean, you already know what they're into.