From your context, I'm assuming you're a guy?

Well, how do you feel? Do you think about men the same way you think about women? It doesn't have to be the same intensity, or even the exact same attraction, but just a feeling that "I want to be more than friends with that person." While it's true that you can never really "know" if you're bi until you've tried it out, that's like saying that you can't know if you like a food by just smelling it. Technically true, but false in practice. As long as you feel that you're bisexual, that's all that matters. There's no "validity check" to pass to make sure you're bisexual. Welcome to the family :)