how do i know?

what most people do for a first step is to take the kinsey test

 

Try google-ing it. The kinsey scale goes from 0 (heterosexual) to 6 (homosexual)
There are tests that you can take that will tell you where on the scale you are and what that means

 

I can't hotlink cuz my browser sucks, but basically what the Kinsey scale represents is on one end, people are completely straight, and on the other, completely homosexual. There are various places in between that people can find themselves, it isn't one or the other. You can look it up on wikipedia if you need to for a better explanation.
Alternatively, look at Jenna Jameson and Cody Cummings. If you get a boner looking at both of them, then you are probably bi. lol
It IS possible for you to have sexual attraction to one specific gender while wanting to have a relationship with the other gender.

 

LOL I never thought I'd see a reference to Cody Cummings here.

Nowadays gender and sexual orientation are so fluid that the binaries have dissolved. Being bisexual just means that you're attracted to both sexes. But even deeper we may consider what types of attractions there are (sexual, physical, emotional). That's actually a lot of go into, but I don't think that sexuality should be something you have to dig deep into and fight with your emotions. When you see a guy or a gal, you'll know whether you are attracted (physically/sexually) to that person, and when you get to know a person, you'll know if you are attracted (emotionally) to him or her. So gender and sexuality are both a continuum, and you just have to experience life and let things come to you to know who you are.

 

I disagree, he may not find both of those people attractive simply because they may not be his type (one Jenna is so sl*tty and fake) so this "test" might give him inaccurate results. I would say with go with who you notice subconsciously on the street the most, If it's both and you have a desire to be with them in a sexual way on any level, then you could be bi, or it could be one or the other.