well...I was in her situation a little over a year ago.
My best friend liked me, couldnt tell me for a while...he probably ended up telling me in the best way I can think...he made it serious and didnt make it overbearing.
He pulled me aside and told me he loved being around me....and that he just straight up had fallen for me. I wasnt shocked really, Girls know things.
I still hung out with him even though I expressed that I couldnt since I was in another relationship. I liked him too...I had a falling out with the person I was dating, but it was still hanging in there somehow...so I had room for crushing I guess. Now here I am, about to have my one year anniversary with my best friend and boyfriend. You just have to trust yourself and trust that she will be straight forward with you and tell you if she likes you too or not.