Each U.S. military branch has its own body composition standards similar to the Army's, below. First, soldiers are screened using a height/weight table. If a person doesn't fit those standards, he or she is then measured at various points (such as waist and neck) to get a more accurate body fat number. The soldier will pass or fail based on that.
SOURCES: Military.com, U.S. Army, Defense Department | The Washington Post - Feb. 1, 2011