The pullets almost always have a darker leg front, but remember, we are talking about today's modern hatchery BRs, too. It may not be as true with show quality/breeder BR chicks. And it is not 100% accurate, plus the color is in varying degrees, as well. If you get a chick that is very dark overall, plus has very dark leg fronts and a tiny headspot, you can be about 95% sure you have a pullet. The old wives tale is the thing about holding by their heads and having them kick or their legs hang.