What does bisexual mean to you, and how do I figure out if I am bisexual? (Not necessarily me specifically, that is.)
Bisexual means to me being sexually and/or romantically attracted to people of more than one sex. I find it irritating that a lot of people who are sexually attracted to more than one sex won't use the label, but I can also see various reasons why this comes about, both reasons I approve of (they are concerned about coopting a label that they think doesn't belong to them and has a frisson of coolness) and reasons I don't (they don't want the negative associations that come with the label; they want to pass and don't care whether that decreases the visibility of bisexual people).
That said, how I began figuring out I was bisexual was by realizing that I fell in love with other women, rather than realizing that I was sexually attracted to other women. See, I knew I found women physically attractive, but I thought all women felt that way, especially given the ubiquitous seductive images of women in all the media directed at women. I had to start openly discussing my attraction to other women in sexual and romantic terms, before I heard some heterosexual women say, "Well, no, I don't feel physically attracted toward other women, and I react to the media images by wanting to be like her, not by wanting to touch her." (I expect they were saying that because they were worried I was coming on to them.)
The other part of figuring out I was bisexual was accepting that I was really attracted to people of more than one sex, even though there were differences in how that attraction manifested itself toward people of different genders, and that I wasn't going to grow up and make up my mind. (I lived on the East Coast in the early 80s and felt a lot of pressure to identify as either het or gay; I felt bi wasn't a politically correct option; I didn't completely come out as bi until I came to San Francisco.)
I dunno if this has a lot to do with how other people figure out they're bi. cyan_blue recently posted an article she had written about it (or was it notes for a lecture?) and it sounds like there are some points of commonality, especially the "not figuring it out as early as het and gay people do" (I was 23 or so when I first suspected it and 29 when I finished coming out).