Well, if its a guy, and he is shanking a woman while another guy is shanking him...
There are no signs. If you are bisexual, no one will know for certain unless you tell them.
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.
'I'm bisexual'
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.
Two ways. You can go up to random people and say 'Hi, I'm a bisexual' Or you can do it subtly. You can find a friend you trust and tell them, or you can let a little conversation be overheard (or a text message read) by a gossiper (the best people for the job). There are also many more ways of doing this.
he is secretly bisexual, don't tell...
If you fall in love with and is sexually attracted to both genders you are bisexual.
You ask her.
There is no real way to tell. The only way to know is to talk to him.
As far as I could tell, there is no indication in any of the ten main novels about him being bisexual.
The best way is to ask them i mean if you are the one with problem ;D