In order to "feather-sex" or determine the gender of a bird by the type and shape of feathers, look for the following: rounded hackle, saddle, and tail feathers on a hen; pointed hackle, saddle, and sickle feathers on a rooster. Males of the Campine and Sebright breeds of chicken are to have feathers similar to the female and are referred to as "hen feathered" breeds. The term "sex feathers" may also refer to the top two curved feathers of the tail of most breeds of mature drakes prior to and during the breeding season.
Figure 1. Location of the different feathers on a rooster. Source: John Anderson, Ohio State University
Figure 2. Pair of pekin ducks showing the sex feathers on the male (right). Source: Jacquie Jacob, University of Kentucky