Outdoor gardeners can take advantage of the results of a 40 year old experiment to discover with certianty, the sex of each plant.This experiment found that the flowering hormone that induces marijuana to flower is locally produced that is each part of the plant produces the hormone (phytochrome), and the hormone is not translocated (transferred or moved) throughout the plant.
Once decent sized branches develop, cover a lower branch on each plant with an opaque (light tight) paper bag or sleeve (not plastic cos the branch must breathe).A double layer black consrtuction paper or a cardboard mailing tube are two possibilities.
Close the end @ a branch tip, but leave a small opening of the paper sleeve or tube @ the other end.After about 10-14 days you should see flowers developing on the coverd branch.the rest of the plant continues to grow normally, with no slowing of overall growth.The prcedure works better for outdoor growers,cos the plants have a longer life, and the branching is better outdoors than indoors.You might start your seeds in March,cover them in may,and you know before june the sex of each plant.Indoors,branches don't form well enough, and the crop doesn't usually have enough time to have any real value.