It is certainly possible to establish the sex of cannabis plants at an early stage. This gives the opportunity to tell the male plants in the growth area by covering the low level branches for up to 12-hours per day throughout the vegetative state. A paper bag or similar covering is certain to be beneficial in permitting the constant airflow while at the same time limiting the light. An ongoing cycle of covering the branches is likely to be ideal.
A magnifying glass can be used in the process of establishing the sex class of the plants. Female plants are likely to be wispy and white with one or two pistils and appear from a juvenile calyx. Male flowers will appear much like a mini-sized club (like the playing card) and minute stem beneath.
A lot of cultivators aim to force the development of the plants until reaching a height of 8 inches to help remove the males. If plants are grown outside, a lot of people prefer to avoid the devoting a significant amount of energy, space, and time on the male version of the plant. Get in the position of placing the plants on a cycle of 12 hours on and 12 hours off for a period of two weeks, and divide the males from the females, then look at reverting back to the ideal light cycle of constant light to maintain the vegetative growth of the female plant. It might be worth noting that early sexing the plants can be a time-consuming process and might delay the growth by approx two weeks. Avoid pre-forcing the plants if you don't have sufficient time. It might be easier to just look at using the black paper bags to cover a single branch on the plants for a period of 12 hours per day to help with forcing the pre-flower stage in order to differentiate the sex.