Which of the following is a correct method to determine if shoulder abduction weakness is due to the serratus anterior or trapezius muscles?

To determine if shoulder abduction weakness is due to the serratus anterior or trapezius muscles, you should feel the tip of the scapula with one hand while testing shoulder abduction with the other hand. If the scapula moves during this test, it indicates that the weakness is at least partly due to the serratus anterior or trapezius muscles. This is because these muscles are responsible for stabilizing the scapula during shoulder movements. If they are weak, the scapula will not be properly fixated, leading to its movement during shoulder abduction.