Pay close attention to the plant nodes, and if you see what looks like little balls which have no hairs protruding from them, you have likely identified a male.
Outdoor male plants will begin to produce their flowers and pollen as early as mid July. Varieties from more southern climates may not start until mid September. This difference depends on the budding cycle of your variety, some plants start to bud earlier than others, so the exact time to cut the males will vary with the strain. If you are using a variety of different seeds it may be necessary to visit once a week from July 21 through September 15. The timely identification of a male plant is crucial to the success of the harvest. If the weather is exceptional during the time a male starts producing its flowers and you missed seeing the first signs during your last visit, you could wind up with a lot of seeds and little of the fine herb. A female can either generate a large seedless bud, a large bud with a few seeds, or a large bud that is almost totally seeds. The first case is achieved by removing all the male plants before any of their flowers open. The second case occurs when a few male flowers have opened but you remove them before any more open. The third case occurs when you miss-time the flowering of the male. This can be devastating if you have big female plants because you could loose 90% of the smokable herb to seed production. This last scenario may not always be bad though. If you are short on seeds for the next growing season, it may be prudent to let one or two males stand and fertilize a portion of the females. Good seeds are hard to come by, so if you have a strain you like, make sure to plan ahead and have at least a few hundred seeds for the future.
A hermaphrodite (sometimes referred to as a hermie) is a plant that develops both female and male reproductive features. Hermaphrodites are generally viewed with disfavor as they will release pollen and ruin a sinsemilla crop. Additionally, the resulting seeds are not desirable since hermaphrodite parents tend to pass on the same tendency to their offspring.
It is possible to tell the sex of a plant early, and thus move male plants out of the main growing area sooner by forcing a small part of the plant to begin flowering. This is done by covering a small lower branch so that it is in complete darkness for 12 hours during a 24 hour period. Use a black paper bag or equivalent to allow for airflow while keeping out light, and be sure to maintain a regular cycle for these covered branches. If light is allowed to reach them during the dark period, they may not indicate early.