On the one hand I think gay folks are much more likely to conform to straightness and have a harder time understanding their same-sex attraction, but on the other hand I don't think the social context can fully dominate your sexuality - if you are attracted to girls, you will have that capacity to feel that attraction innately and it will be there even if you don't understand it exactly.
So how do you know if you are bi? Are you able to feel sexual and/or romantic attraction to men and women? That's about it ... the ability to feel that attraction is what's relevant. Straight people can't feel same-sex attraction, they only feel attraction for the opposite sex.