Jaguar what you said isnt true believe it or not. Cannabis plants do not grow the exact same on every branch and not all the flowers on the plant will flower and finish at the same time. Covering one of the branches WILL trigger flowering on that branch only. The plant can in fact segregate branches and growth tips, this is why cloning with branches is possible. It is however, one of the more difficult ways to sex a plant.

The easiest way to sex a plant is to Top it. Topping takes a little longer to show the sex of the plant than the covering method, but its more safe and works the best. Topping not only sexes the plant, but it increases the yield and keeps the plant from getting too tall.