I usually recommend developing platonic friendships first, romance later. That way, you can talk about polyamory before anything gets romantic, and the other person will have already had time to think about it.
I doubt there's any easy way to introduce someone to poly once you're already dating them. You just have to say (and you should say it within the first few dates, or even on the very first date), "I should let you know that I am polyamorous. Is that something that you'd be okay with? Are there any questions I can answer about it?"
OKCupid is a good dating site where you can write up a thorough profile page for yourself, and explain right there that you're poly. Not everyone has good luck with OKCupid, but many people do.
You can also google "polyamory" with the name of your state or city (if it's a large/major city) and see if you can find any poly organizations in your area. Perhaps there are poly people reasonably nearby who get together for social events.
Oh and check out our Dating & Friendships subforum
Poly people come in every size, shape, and style, so I wouldn't bother looking for some special code in someone's behavior that clues you into whether they're poly. You just have to bring up the subject of polyamory, and ask them what they think about it, and find out if they're poly that way.
Hope this helps.