LOL... for me, when it comes to preference (gender, "kink" {if there is any}, and number of possible partners) I wish it was a plain as telling "male" from "female"...
You know, it would be nice if people could tell from 20 feet away "that person is bi and poly", "this other person is gay and mono", "that person over there is straight and likes to be the center of gang-bangs", et cetera... but that would only work in a world where everyone is accepting of all "flavors".
So, I wouldn't want a "secret" handshake. It would make it much simpler if it was as plain as the nose on one's face.
Actually poly does have an official symbol and an official flag. I think you can buy the symbol as a pin you can wear. It's an infinity sign superimposed over a heart figure.