Need help with an important question?

I have a friend who is having trouble trying to figure out wether or not if she likes just guys or just girls. Here name is Sara. She had just recently told me that back in her child hood she had some pretty bad things that had happend to her that might be causing her to be thinking this way. and she ask me how does she tell if she is byual or not. so i'm asking any one who would know any answers to write me back so I can answer my friend Sara in the best way possiable. Thanks Jess.