Now that your plant has started growing, it’s time to determine the sex of your plant. For those looking to grow THC flower, a female plant is needed. If making feminized seeds, or developing cultivars, then the male will be important. Typically cannabis consumers are looking for the female plant with their frosty THC coated flowers. A cannabis plant will show it’s sex in the pre-flower stage, in as early as three weeks and potentially out to six weeks, dependent on a variable of factors. Identifying the sex of your cannabis is really important. Male plants, if allowed to grow until flowering, will pollinate nearby female plants, turning them into either males or hermaphrodites. This can devastate growers’ hard work, and often can happen even in the most controlled environments. If growing outdoors, pollen from male plants can travel several km and pollinate other outdoor female plants (it happened to me last season). Even indoor grows can become susceptible; if you have pollen on your clothing from being around male plants, you can still pollinate your female.
Basically, when identifying male vs female in pre-flower, look for the pistils extending out of the calyx to indicate female. A small sac will identify males. Again, if you are looking for feminized plants, destroy any males IMMEDIATELY!
Here is a great resource that identifies how to determine the sex of a cannabis plant in pre-flower.