I guess you can claim that you aren't certain if you are truly bisexual until you have tried sex with members of both sexes, but unless you really don't know yourself or like to lie to yourself a lot, I would guess you can have a pretty good guess?
This is an interesting subject that I've given a lot of thought to. I have found that while I am not sexually attracted to a member of the opposite sex (or rather haven't found a guy yet that I'm sexually attracted to), I often am emotionally attracted to the other sex. I wouldn't be opposed to...