Determine the intended audience for the song you wish to classify. For example, consider if the song is intended for mass or private consumption, as you can classify a song intended for mass consumption into sub-categories: artistic or popular appreciation.
Position the song in a timeline of related songs. For example, music theorists classify orchestral songs from the 1600s as "baroque," while orchestral songs from the 1800s are considered "romantic." Similarly, you can categorize improvised horn songs from the 1920s as "swing jazz," while improvised horn songs from the 1960s are classified as "bop jazz."
Identify any geographical variations that exist between a song and related songs. For example, [Mississippi] Delta blues songs feature a focus on the singer and his or her guitar, while Chicago blues songs focus more on the singer and a band, including a guitar, bass guitar, drums and, occasionally, a harmonica.
Highlight various musical techniques employed by the artists in the song. For example, music theorists classify singing without instrumental accompaniment as "a cappella" singing.
Trace any hybridized elements the song's performers/composers might have employed in the creation and recording of the song. For example, Miles Davis' "Pharaoh's Dance" represents a fusion of rock, R & B and jazz stylings.
Identify any social purpose the music may play or may have played in the past. For example, Woody Guthrie's music was incredibly influential as protest music, while the music of Aaron Copland -- a contemporary of Guthrie -- was often acknowledged as patriotic.