i started writing you a response, but i soon saw it was getting lengthy so i wrote it as my own confession--15232.
since jane and i started to be intimate, i've noticed this about other girls:
1 - how do they talk to you, flirty.
2 - they check you out.
3 - they are more touchy when around you.
4 - they are around you for no real reason
5 - do you look up and catch them looking at you and they smile?
with jane, i don't think any of this happened. she was not looking for a bi encounter and we were both straight. so barring the awkward event that took place with us, i'd have to say simply ask. like when you're talking about sex you might add in something about a bi story or image you saw and if she had ever run across any and what she thinks about it, or maybe as if what she would do if she found another girl coming on to her.
in any case, good luck with what you decide. i know my story is very awkward, but i so would go through that embarrassment again to have things turn out like they did.