Prior to my conversion to Orthodox Judaism, I sought as much sex as possible (without spending much time, money, effort or dignity), but I’ve always hated illegal drugs and have shied away from people who indulge in them.
You might think it’s all about personality, and ask about their “openness to new experiences” or some such. You could go the religious route, and ask about how often they go to religious services…
But here’s the best one: “Is sex without love OK?”
If they answer yes, it is OK, then you can predict with a certain degree of accuracy that they will be in favor of legalizing pot. If they answer no, then, obviously the reverse.