I remember ages ago, during a homestay overseas, when my host mom kept being asked whether I was a boy or a girl. That felt wonderful looking back on it. It made it almost feel like my gender wasn't written in stone, that I had freedom. I remember just sheepishly not answering, letting my host mom do the talking. That wasn't very nice of me, I know...:-[
Now, I rarely have people ask. They just assume one way or the other.
Actually, the few times people have asked over the years, I seem to remember always responding "It's complicated" or even more upfront "My genes say male but my brain says female". These went over differently with different people, but then, that was back in the day. Not many knew about trans folks.
Whatever you do say, say it with a smile, and be genuine when you say it. That would be my advice.
