Do lesbians really do this: Lesbian Flagging Codes?
This is usually called the hanky code and while some lesbians may use it, it seems to be most popular amongst gay men and the BDSM community. The idea is that you wear a handkerchief or bandana on your wrist or in your back pocket with whatever type of sex play you are interested in for the evening. This can make things a bit easier if you are a kinkster on the prowl and want to up your possibilities of participating in something specific and very kinky that night. Imagine if you had to flirt with every bar patron for 20 minutes each just to find out that only one or two were interested in the same kinks you are. This saves you time!
Some bars will even have special nights dedicated to the hanky code where patrons are encouraged to wear their kinks on their sleeve, or butt as it were.
I say it is a win, win! Not only do you get to figure out what your potential play partners are into, you have an excuse to check out their ass too.