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.
Babies though are more problematic, since the secondary sexual attributes haven't yet developed.
And names aren't always a good indicator, either, since parents can tend to give their children the most amazing names for either gender, usually with dreadful intricate spelling as well.
I've had a little boy in my class called "Jade" and three kids in one class all called "Lindsay", one of which was a girl, the other two boys.
That's not counting all the various ethnicities to negotiate, with names such as Dinoga, Disha, Remi, and Millar. Often the result is quite unpronounceable, much less giving any indication of gender!