Women, as a gender, are not struggling for more ways to make the male half of the world see them as being sexually available. I recently had cause to remark upon this fact to a straight male friend after he innocently remarked to me, "I wish there were a hankie code for straight people."
He's not the first man to yearn for a heterosexual system analogous to the one invented and utilized by gay men beginning in the 1970s, where colored bandannas worn in the back pocket of one's jeans indicated sexual availability and tastes. The practice of "flagging"—famously fumbled by Al Pacino in Cruising—was designed to facilitate anonymous sexual pickups. "I feel like sucking cock tonight—hope I find a guy with a light-blue hankie in his left pocket!" The list of what color supposedly goes with what activity has grown to comic proportions, so you'll have to Google it if you want to try this. Here's a hint: Don't wear a brown one on the right unless you've really done the research.
Whatever color you choose, you'll have to experience it with other guys, because women don't flag. Oh, occasionally one does see a woman at a BDSM-oriented event displaying a hankie—but she's usually queer, and she's usually doing it as an homage to the gay leatherman tradition rather than a practical means to an end. In my experience, it's rare for any woman to flag with the hope of meeting and immediately consummating an erotic experience with a strange guy.
Do these guys wishing for a hetero hankie code really think women walk around longing to have sex in public restrooms, and that we are only stymied in this desire by our inability to broadcast it to passing strangers? Of course not. (Although some of them would certainly like it if we did.) What I interpret them as saying is "I wish I knew if it were cool for me to just talk to a strange woman." Sometimes it means "We're acquainted, but I'd like to be more flirtatious." I think it's great that they don't want to be creepy—far too many men have no such qualms. And, unfortunately, respectful guys get stuck with the fallout from that.
So embrace the mystery, boys. Women aren't straightforward and practical about getting our sexual partners quickly and efficiently, because we do not wish to be. As long as there's no universally understood and adhered-to hankie that means "Nope, I'm definitely not available," women aren't going to wear one that unabashedly signals "Yes!" But take heart: Your situation isn't nearly as dire as that of gay men in the 1970s. The hankie code was widely adopted because in a homophobic world, hitting on the wrong guy could turn out very badly. I admit I've dealt out some withering snubs in my life, but I've never actually bashed anyone for simply making me an offer.