Look at the flower buds, they will appear in between branches. Females are easily diagnosed since you will be able to see the white pistils coming out in an upward position.
Meanwhile, male flower buds tend to be smaller and are hanging downward like... testicles! (In fact, flowers are the reproductory organs in plants).
Always keep an eye on your marijuana plants as hermaphrodites tend to appear. You can't be for sure until the plant is fully mature. Hermaphrodites have both kind of flowers, but they grow and mature their female part early enough to minimize self-pollination.
: "white pistils" refering to a young female plant, they will turn brown as time passes.