Clinical testing of the function of the long thoracic nerve is done by:

Injury to the long thoracic nerve or nerve to serratus anterior leads to inability to raise the arm beyond 90 degree i.e. overhead abduction. Serratus anterior rotates the inferior angle of scapula laterally during abduction of the arm above the head. Winging of scapula is another classical feature of long thoracic nerve injury. Ref: Clinical Anatomy (A Problem Solving Approach), By Neeta V Kulkarni, 2nd Edition, Page 95, 96, 107-109, 114, 115 ; B.D.Chaurasia's Human Anatomy: Regional and Applied, Volume One, 3rd Edition, Page 39, 40, 44, 45