I am always hearing about how difficult it is to find a Bi girl for a relationship or sexual encounter. Even on sites like this one where we all admit we like other girls.
So here are my thoughts and experiences.
Do you approach someone when you don't know for sure if she could be bi? Sure! The only way is to dive straight in. Nowadays being gay or lesbian is quite acceptable to most people and especially females. Look the worst that can happen is they give you a dirty look or tell you to go away but most straight girls will just say no thanks or I am not like that, sorry.
Why not try and go somewhere you can ask someone to dance. This usually works for me.
There usually arent enough males to ask to dance anyhow so an opening line might be. Hi, how about dancing? None of the guys are asking so how about it? Better than sitting alone all night. Anyhow you look a lot more attractive than any of the guys here..... laugh. ....
Also try talking to your friends. I discovered that many so called straight girlfriends are actually also bi or at least bi curious. But that most of them are very shy and will not let anybody know.
Anyhow, in my opinion, we have it pretty easy as a bi girl, and I would certainly advise you not to be afraid or feel stupid when you flirt with another girl - you have a 50/50 chance in my experience. And no matter what, you will have a fun and sexy conversation.