I remember going through a phase where it felt like every single girl I was interested was already seeing someone. "Someone always beats me to the good ones!" I used to lament. I even remember a couple times a girl would be by herself or with a friend I'd start talking to her and the guy would walk up, with an expression of either "haha loser, I got her and you didn't!" or "get away from my girlfriend or I'm going to stomp you into the ground".

Are there any tricks to singling out the women who don't already have a boyfriend? I wish they'd wear rings like they do when they're engaged or married.

Well, when a girl says, "I have a boyfriend" it doesn't necessarily mean they actually do - it can mean they're not attracted to you.

Just approach more women, and get used to the fact that sometimes you'll have to deal with men protecting their gfs from you.