Ooh bigender, that's an old one. Let me sweep the dust from my memorycloset first...
First off; the reason why there isn't a bigender forum is because we're one big, bunch here that use 'androgyne' more as an umbrella term. So there's androgynes, genderfluids, neutrois, nullgenders, bigenders, some that define as third gender... There's also male and female identified genders that simply don't feel a fullblown man or woman... Really, it's one big party...
Now to answer your questions about Bigender: some who is bigendered feels male and female at seperate times. They're never in the middle but switch on the binary between left and right. As to how they would know... That's one bettere answered by someone who identifies as bigender. However, I'd suspect they feel 'completely' male at one point and 'completely' female the next.
Often confused with bigender is genderfluid: an individual who dances across the binary scale. They glide from male to inbetween to female and everything between that so there's no real 'switch' like with bigendered. I'm sure there's different bigendered folk out there who all have their own way of dealing with their identity.
How would you know if you're only one gender?... I guess you can better revise that as 'how would you know you're not two genders?'... wait whaa-... Well that makes the question easier to answer. Someone's who identifies as bigender doesn't identify as both male and female - they identify as bigender: an harmonious marriage between two binaries in one gender. Ehhh?... Well I never said my answer wouldn't be confusing..
Personally I know I'm not male, I'm not female and I'm not inbetween. I'm nullgendered, a selfconstrained neutrois. I'm androgyne. Take a transsexual for example... They know they're not their birthsex gender wise. It's a feeling I share. It's where the two come together androgyne and transsexual. Yet where they come together they also split again for one finds the opposite of their birthsex their true gender while for the other that same initital feeling 'this is not my gender' is repeated. You have those Androgynes who feel fine there, right in the middle, those who wander from left to right, those who teleport from left to right, and those who go 'screw this' and jump from the binary scale altogether. In the end there's only a sense of self, of right and wrong - simple feelings.
And that concludes my wall of text.