The color/pattern is too uniform for a male, and a male at 12 weeks would have a very red, well developed comb, deep rusty red through the shoulders, and visible male saddle feathers. And 99% of birds sold as Ameraucana are actually Easter Eggers.
Black sexlinks can be sexed by color. Males are barred like their Barred Rock Mothers. Females are black with red/gold on the head, neck, and chest area. The amount of red/gold can be highly variable.