It is fairly easy to differentiate between male and female chickens when they are adults. Depending on the breed, adult male and female chickens have different feathers. The males have pointed feathers around the neck, back, and tail; in females these feathers have round ends. If the chickens are purebreds, the coloring patterns of the males and females will also differ. In addition, males typically have larger combs and wattles and large spurs on the back of the shank (leg). Male chickens crow, and female chickens do not.
The Sebright and Campine breeds of chicken are exceptions to the norm. Male Sebrights and Campines are feathered similarly to the hens and are said to be "hen-feathered." For more information about hen-feathering, refer to the article discussing the Causes of Hen-Feathering in Male Chickens.
Comparison of the parts of male and female chickens. Source: University of Illinois. Used with permission.