It is possible to tell the sex of a plant early, and thus move male plants out of the main growing area sooner by covering a plant is lower branch for 12 hours a day while it is in a constant light vegetative state. Use a black paper bag or equivalent to allow for air flow while keeping out light. Be sure to set up a regular cycle for these covered branches. If light is allowed to reach them during the dark period, they may not indicate early at all.
Use a magnifying glass to look at the early flowers sex type. A male plant will have a small club (playing card) looking preflower with a small stem under it. A female flower is usually a single or double pistil, white and wispy, emerging from an immature calyx.
Some people like to pre-force plants when they are 8" tall, in order to weed out the males. When growing outdoors, many growers do not wish to devote time, space or energy to male plants. Just put the plants on a 12 hours light cycle for 2 weeks, separate the females from the males, then revert the light cycle back to 18-24 hours to continue vegatative growth for the females. Keep in mind, this is a time consuming process and can put the plants back 2 weeks in growth. Do not pre-force plants unless you have lots of time. Just cover one branch per plant with black paper (light tight, breaths air) 12 hours every day under constant light to force pre-flowers and differentiate early.