Bracelet, necklace, pinkie ring, secret sign. We all talk about how to spot swingers in normal every day life. Perhaps if we all just bought one of these 
http://www.engadget.com/2011/12/07/magnetu-connects-you-with-similar-people-in-your-area-because-d/
Seriously though, it would be horrific if one was to run into their boss at a bar who for whatever reason also happened to be carrying one of these devices while yours was set to swing. But none the less, it was the first thing I thought of when I read this article.
http://www.engadget.com/2011/12/07/magnetu-connects-you-with-similar-people-in-your-area-because-d/
Seriously though, it would be horrific if one was to run into their boss at a bar who for whatever reason also happened to be carrying one of these devices while yours was set to swing. But none the less, it was the first thing I thought of when I read this article.