If you don't know the gender of a grownup, do you call the grownup It? No, you don't. So why do this to a child?
Adam's Apple, prominent eyebrow ridge and jaw, facial hair, deep voice, wide shoulders (compared to hips), and/or an obvious 'package' generally means male.
No adam's Apple, smooth forehead, receding lower jaw, little or no facial hair, high-pitched voice, narrow shoulders (compared to hips), and/or prominent 'booty' generally means female.
If I just can't tell, and I need to interact with that person, then I usually go by the name, which usually (but not always) gives a strong clue.
If no interaction is needed, then I wait for someone to clue me in. Otherwise, knowing a stranger's gender isn't important to me.