Through their maturity levels and their genitalia structure. Gender is determined by chromosomes A male has an X and a Y chromosome. a female has 2 X chromosomes
He's a boy, though in the 2003 TV series, he has a little sister built by Dr. O'Shay, named Zoron. Her name was Uran in the Astro Boy manga.Well, judging from the name Astro BOY, he's probally a boy.