i've been asked this before, not because they couldn't work it out themselves with my appearance, but because my behaviour did not suit a girl at all. i remember when asked this at age 12 i said i was ...- an offensive word used for people who don't clearly identify as their born gender, and i didn't know it was offensive. and i didnt know what else to call myself, since i felt like i was a boy but my body suggested otherwise.
3-4 yrs ago if somebody asked me that i would have said 'i don't know really. and i don't care. so i don't think anybody else should care'
right now if someone asks me i'd say 'what makes you doubt that?' after they reply i'd smile and say 'take me as whatever you feel i am, not necessarily what my body looks like'
some day in future if i live fully as male i'd give a confused and annoyed 'huh?' with an 'are you insane' look.