Funny that someone would mention this that is presumably experienced in the lifestyle. The reason I say this is that I have yet to meet anyone who has been swinging for very long that "secret signals" have actually worked for. The fact is, like a lot of folks, when we were new, we tried this and it didn't work very well.
Like most of the others that have responded here, we use the direct approach. Once we have met someone and determined their is a mutual interest, we excuse ourselves and discuss whether we are both on the same page privately. We don't even hide anything from the other couple. We usually say something like, "we are going to discuss this privately, be back in a minute". Once play starts we are equally as direct, if things are not going well for either of us, that person will stop and say so to all involved, and we will politely exit. That hardly ever happens, but it does happen, usually because of lack of communication of what is expected before we get to the play room. But sometimes it is also just that the person you thought would be compatible with you on first meeting turns out to just not do it for you at all in the bedroom.
Point is, secret signals and such just lead to confusion, direct communication is always best.