Well, there're two ways to pass for a guy, really. One is in the face, the other is in the body. I think in your case, it's more that you have a boyish face, and in my case that I have a boyish body. Someone could take one look at you and go 'ah, a boy!' then look a little closer and realize their mistake. With me, it's more 'ah, a g- oh, wait, no it's not... or is it?'- since with all but the tightest of shirts, I have NO visible cleavage, they either have to assume a pretty man or a woman with no breasts. It's interesting to see which they choose.