Prelude: If you don't "get" or "approve of" polyamory, please just don't say anything to this thread. I don't want to hear it. Okay, so my wife and I are polyamorous. Happily, faithfully polyamorous. But it's really hard to talk about. Neither of us are really looking for anything serious, but would love to go on a few dates, hang out with some cool (and also hot) poly people. But being polyamorous is something that is really hard to bring up in casual conversation with someone you think is attractive/interesting. So is being married, for that matter. Poly people: How do you do it? How do you broach the subject and poly-date?
Personally, I would be upfront as soon as possible. Definitely say something before a date so the other person doesn't get misled, but you also don't want to be so direct it scares them off regardless. So basically the best option is to have some small talk/minor connection before doing so. There are also dating sites where you can put it on your profile. I think there is one that actually asks you about your "relationship preference". I understand that even with this advice, it can be hard to do since you don't know who will get offended. But the people who would have rejected you are better off not dating you early on rather than later on in the friendship.