How can you tell if you have a winged scapula or not?

Physical Examination. A winged scapula can be diagnosed by an orthopaedic surgeon. Presence of a winged scapula may indicate underlying shoulder pathology that may need treatment. Recommend you seek a consult for evaluation.
Test for it. The easiest way to test for winging of the scapula is to have someone observe you from behind as you lean forward and push against a wall. If the scapula sticks out compared to the other side you have winging of the scapula which has dozens of different causes from the benign to the serious.