If grown from seed, marijuana plants emit preflowers around the 6-8 week mark..depends on genetics and environmental conditions as to exactly when. Your a bit early to probably see any signs yet, but should be seeing some in the next few weeks.
The pic doesn't show anything to me as far as the plant's sex yet...give it time. You will see two white/yellowish hairs start to protrude from behind the stipule (spurs) that poke out at the plant's internodes. When you see these, this indicates the plant is sexually mature and is female.