Sexing Marijuana is only necessary if you choose to grow from seed. Cannabis grown from seed needs to go through pre-flowering to determine the sex of the plant, either male or female. Males produce pollen and seeds which can and will destroy a harvest. Female plants produce marijuana buds. Counting up from the bottom of the plant about 6 nodes, you will see the pre-flowers, each sex having its own distinct flower. Males need to be removed ASAP or else your harvest will be ruined. If you are trying to cross-breed strains, then you will want to keep the males and their pollen.