How do i know if she's straight or not?

I'm still in high school, im young yes, but. There is a girl who is in a couple of my classes. I have listened to what she says and she seems really sweet and whatnot. Shes also in my opinion one of the hottest girls in my grade. Now i would totally talk to her but im shy and shes in the 'popular crowd' and im not. not to judge but she is one of the few girls in my grade who dresses the part of a lesbian. shortish hair cut, baggy jeans and such. Ive asked her before if she was and she said no (on a fake facebook account a year ago) but i dont know what to do? i really wanna talk to her. ugh shes so damn cute! thankyou(: