There's just one question you have to answer - What turns you on more guys or girls? I mean really turns you on? Which gets you hard and makes your want to "adjust" yourself?
If you're really not sure, you can try having sex with both. Then, ask yourself the same question.
If you're still not sure, or if they both get you going, you're probably bi.
If you're not in a logistical position to make the decision, you're not ready to make it.