Mike Solomon demonstrates how he tests for a weak infraspinatus. The infraspinatus is an external rotator of the shoulder. When the shoulder is at 90 deg. abduction, and the elbow flexed, the patient cannot hold the arm up, especially when her attention is distracted - see video clip.
This test must not be confused with the drop arm test for a massive tear, which tests mainly the supraspinatus. In this test the elbow is kept straight and the arm lowered from above the head to a 90 deg. position. Again, when there is weakness the arm fells to the side.