I'm always a fan of simply asking.
The best way to find out what somebody thinks about any given topic is to ask them about it.
It doesn't have to be any Big, Serious Talk (tm)--I'd treat it like any other discussion (like, say, asking about what kind of car you want to buy next). Just ask "Hey, have you ever thought about us having a poly relationship with other people?"
When speaking of various forms of non-monogamy...it ain't poly if you're just fucking around.
While polyamory, open relationships, and swinging are all distinctly different approaches to non-monogamy, they are not mutually exlusive. Folks can, and some do, engage in more than one of them at a time--and it's all good.