I know exactly how you feel. That was definitely me when I was younger (not going to say how *much* younger - LOL). I had some multi-partner experiences when I was in college and for a little while after, but they were mostly sexual circumstances that didn't leave me with any perspective on whether I would want to have a relationship with a woman.
My circumstances have prevented me from dating women, so I had to come to my present conclusions without any empirical evidence, so to speak. You can't look to your sexual fantasies for the answer to this question, although the length of time you have been having them may say something in itself. To me, being bisexual is a state of viewing the world without a certain filter - the one that tells you what's between a person's legs. In my mind, asking if I prefer to date men or women is similar to asking whether I prefer blondes or brunettes; it doesn't matter as long as it's the right person.
And, of course, it's perfectly ok if you're just curious right now. There's no deadline to make a decision, and there's no reason you need to bring it up to anyone. I've only recently been freed to express who I am, and I still feel unsure about taking the step, even though I am now sure about my orientation; I certainly don't need to declare it from the mountaintops or even tell relatives or friends who I don't believe will understand. Take it at your own pace, and don't ever let anyone tell you that you *need* to be one way or the other or, if you decide you would like to explore being with women also, that you *have* to come out in order for your feelings to be real.
As to how to figure it out, that's the million dollar question that thousands of people are still asking themselves. I don't think it's the same for any two people. The only advice I can give you is to keep your mind open and really see the people around you for who they are instead of what gender they are. I can also tell you that, for me, it was not a decision I made when I woke up one morning, nor was there every a moment of epiphany when I "just knew." The realization came over time as I became more and more comfortable and confident with who I am.