I would say that not many people let alone parents would guess that a girl wearing boy's clothing and making herself look androgynous or masculine would mean that she is trans. (F pronouns used here from Mom's perspective). Most, if they had to guess, would think tomboy, or possibly lesbian. And most parents likely don't want to admit their daughter might be lesbian, either!
Some parents might mistake the androgynous/masculine clothing for not caring about appearances and just being sloppy, or maybe even depression. Trans issues aren't at the top of their list!
So I think you're going to have to come out if you want her to understand that you're not a tomboyish girl; you're actually a guy.
It's scary, but we've all been there.
Jay