That early does not necessarily mean boy or girl. Basically they are all born with tiny little combs. As the chick gets older the size will increase and at certain ages comb size can be one of the things to look at when determining the sex of the bird.
I guess just through experience, but I find the BR to be one of the easier breeds to sex at hatch, and/or again at 5 weeks.
At hatch the male has an irregular, larger head splotch, while the female has a smaller, circular head spot.
Many females has black wash on the leg front, while the males oft times have less of this marking. Not 100%
As they feather, the male has much lighter fuzz and the white barring comes in much more vivid. The over all appearance of the male is thus, lighter, while the pullet is darker in over all appearance.
The males will sprout bright red combs and little starter wattles at 5-6 weeks, while the pullets will show no wattles and combs generally remain yellowish in color.
Sexing a BR is never 100% accurate, but it sure is easier than many breeds.