Being bisexual

Being bisexual is not something to be made light of. It can be extremely confusing and heartbreaking, and it might be difficult to deal with emotionally. A bisexual woman is not there for your erotic entertainment, so if you want to spend your time with a bisexual woman, get that out of your head -- this is not what this relationship is about. If your girlfriend comes out of the closet, treat it the same as if she told you shes a lesbian: with kindness and care. If she decides that she would like to experiment with you and others, good for you both. If not, dont push it. 

She openly appreciates women

Women who love other women and can appreciate their finer qualities are gems -- they are generally not threatened by the beauty of another, and can join you in admiring them. Everyone loves pretty things, and women overall tend to be very pretty things. This is certainly not a definitive clue, but if she is able to appreciate the sex appeal of a woman, then it means her mind is open to observing such things and maybe, just maybe, its also open to more.

She refers to past experiences

If she gets with girls -- or has done so in the past -- it is likely enough that it will get bought up at some point. With all the woman-smooching thats been going on in the last 10 or so years (apparently everyone is doing it or has done it), it is highly likely that she has done it at some stage. Whether she liked it or not, and wants to repeat the experience, however, is another story. You will need to probe for more information.  

She's into girl-on-girl porn

Her loving lesbian porn is a dead giveaway. This will tell you a lot more than any other clue. If she can actually watch and enjoy a woman being pleasured by another woman, then it is highly likely that she wouldnt mind a bit of that action herself.  

She's overly affectionate with certain friends

This can be a good clue, but dont let your imagination run too rampant -- it probably isnt the girl you wish it was, and making up huge fantasies about your girl and her friends is likely to land you in trouble. Hinting will probably get you in trouble if youre wrong, so keep it to yourself until you can see what is clearly going on.