I'll float the idea of drug use out there in some kind of general way that doesn't implicate myself directly, then (based on their response) I'll either continue developing the topic or I'll drop it. That's if I even choose to bring it up though, which is unlikely most of the time. If I happen to have access to some good droogz and I sense that they're the type to indulge, I just might though lol.
Generally I'm of the belief that the less people who know the better though. It's one of those things that seems highly dependent on the individual person. If you're unsure about someone and want to test the waters there are ways to do it, something the members of marginalized groups (drug users, communists, homosexuals etc.) have long known. And it's not even in the sense that you're "hiding" something from a potential partner..even if you are a former drug user, I personally would be discriminatory with who you choose to share that info with, at least until you get a better sense for who they are as a person. That's just my own perspective on it, though, I think that perspectives on this will be highly dependent on the person in question and it's all equally valid