Determining the gender of young cannabis plants is no simple task - in spite of the grower’s pressing need to oust those roudy male plants before they can pollinate the females. Sexing while the plant is still in it’s growth stage, before the ‘flower now’ cycle has begun, is a skill which must be acquired (as well as magnified… cannabis’ naughty bits are really really tiny at this stage).
Sexing when the flowering starts is easier - the male flowers look like little round sacks, (or, frankly, like balls) while the female pistils are slender, graceful tendrils which capture the floating pollen spread around by the males in a sticky grasp. If you want true sinsemilla - female plants grown without pollination - then you need to separate the would-be love birds before accidents happen.
You can force the issue by manipulating the amount of light your plants are getting - lengthen the exposure to light and your plants will reveal themselves - but this is difficult to manage properly with outdoor plants. Also, Indica plants occasionally get stubborn and won’t return to the growth cycle when normal lighting is restored. Please note that this doesn’t apply to clones as they are always female. You can also use blue light to partially force flowering without your plants being forced out of the growth phase - this works best for larger numbers of plants.
If you separate early, you run the risk of discarding a few gals along with the guys, while if you do it after flowering, you run the risk of acting too late and watching your crop become a seedy mess of mediocre marijuana. Even after removing the males, you have to watch out for hermaphroditic off-shoots on the female plants.
Fortunately, technology is revolutionizing sexing your cannabis crops, offering foolproof DNA testing from tiny samples of plant material. This offers more than just quick and efficient gender testing, but also provides info regarding the THC to CBD ratio, among other things, again allowing the farmer to put his energy into plants which provide the crop he’s looking for. Some resources for DNA sexing are: