12 is pretty young to figure that out at this point. Wait a few years and it should be fairly easy to know.
There is no real way to tell. The only way to know is to talk to him.
There are no signs. If you are bisexual, no one will know for certain unless you tell them.
'I'm bisexual'
If you fall in love with and is sexually attracted to both genders you are bisexual.
You ask her.
As far as I could tell, there is no indication in any of the ten main novels about him being bisexual.
There is only one way to tell: you have ask her.
There is only one way to make your parents believe that you are bisexual. You will just have to tell your parents.
You tell someone.
I would tell your bisexual friend that even though its hard to tell them but you have to because you're his friend. then nicely tell them that the other guy isn't interested and that there are better people out there for him.
The easiest (and only conclusive way) to tell whether someone is gay or bisexual is if they tell you themselves. Despite all the stereotypes and generalisations that we use, a person's sexual orientation is only known to them.
The only way to know is to ask your partner. There are no tell-tale signs.