I'm not sure if there is a 'right time or place' ... It really depends on the skill level of the guy who approaches the couple!
I've done it plenty of times here in Austin.
Few months ago I started casually chatting up a young couple with a cute girlfriend in the grocery store. Turns out they have seen me around before and I simply ask if they ever want to hang out for drinks
we exchanged info and before you know it they were at my place having fun
it helps to be a natural talker... A 'people person' in general. That way you can gauge the conversation and see look for subtle hints or Clues. Nobody's going to break the social contract and just come right out and say "hey I'm a swinger are you two swingers??"
But there are certain body languages that a skilled guy can read into to tell whether to push or not.
For me, I pay attention to the way a woman is dressed and I pay attention to her eye contact. I always give respect to the man first and address him first and then I include them both in the conversation so all three of us are engaged. Rather than ask a bunch of questions like I'm interviewing or probing, I will share some stories and throw in a few dirty words here and there to see how they react.
In my experience, open couples and swingers well reciprocate the light dirty talk or laugh in agreement.
Once they start sharing their private life and asking more about Me... At that point I know I'm onto something!