I am not sure you can actually find out if someone or a couple are poly unless you are aware if either of them are in a relationship with someone in addition to their spouse or just asking them straight out.
Sounds like all 4 of you are very comfortable with each one flirting with the other SO and it sounds like you all hangout with each other a lot in a friendly setting. Would you and your husband be comfortable enough telling the other couple that you are poly, but without expectations of developing a relationship beyond the friendship you all have?
Or bringing poly up casually over drinks, say you and your husband are considering it and ask the other couple what their thoughts are on poly relationships are, just to see their reaction. If you feel a sense of comfortableness on the subject, than just kind move onto a new conversation.
I do have to admit, from all the flirting that goes on between all of you, bringing up the topic of poly may open all new doors for you all. Might be worth talking about.