Here are two of my SLWs... I have three they are almost 10 weeks old now... These photos were taken about a week ago when they were 8.5 weeks old... That was when I knew for sure about genders.
As you can see, both have the white shoulders, and the one in the back has an enormous set of waddles already... but look at the tails...
The one in the front is long and "pointed" and has a prominent amount of white on the long straight feathers on the top of the tail. This is a pullet.
The one in the back is more "square" and is mostly black, but some of the newer feathers coming in are GREEN. This is a Cockerel.
You might also note the more prominent black ring around the neck. Also... keep an eye on the feathers on the back...
Pullet (See the uniform shape and color of feathers?) Roo (see the pointier white fringed saddle feathers? Also, the Green iridescence)
They will be 10 weeks old on wednesday, and the cockerel's tail has filled in to be almost as pointy as the pullet's, but his tail feathers are emerald green and not white with black lacing.