You should know within a couple of weeks, whether it is a BR or a Marans. The BR legs should take on a yellow cast fairly quickly. The sex may take a little while longer. On the Barred Rocks, I can tell the sex either by head spot at hatch, or just by looking at them at the 4-5 week mark. The cockerels are very bright with wider white bars. The cockerels will sport red combs and little red wattles at 5 or 6 weeks which is a dead give-a-way. The pullets have noticeably darker appearance.
Here's to hoping you have a Marans pullet.