Don't know what breeds you have, but single comb breeds pretty much have a common trait. At 6 weeks, the cockerels will sprout red combs and wattles, while the pullets don't normally sprout these characteristics until much later, closer to maturity.
Thus, sexing the birds at 6 to 8 weeks is pretty straight forward. The tail isn't the tell tale sign that a lot of folks think. The cockerels won't show that kind of typical rooster tail until much later in life, close to 5 or 6 months of age.
Once you get used to seeing the difference in the cockerel's stance and strut? That too is a sign. Cockerels often have thicker legs and larger feet as they will become larger birds than their female counterparts. As the cockerel matures, his feather will become exotic as well. He'll flash saddle feathers, which form in front of his tail and drop like saddle bags, thus the name saddle feathers. The head shape of the cockerel is also different. It is squarish, I guess you'd say. He stands erect and is watchful. His upright stance gives him away. They crow as young as 9 weeks, but many will wait until longer, especially in the presence of older birds.
Since your birds are now 13 weeks, you'll know what you have very soon.