How do you identify a rotator cuff injury?

Strength Testing. There are a series of tests a doctor can use to determine which, if any, of the rotator cuff's have been injured. In most of these tests, the physician will ask you to hold your arm out in various positions and ask you to hold it up against resistence. Pain, or inability to hold up the arm may be an indication of injury. Also, an MRI can help determine injury as well.
MRI or ultrasound. After an orthpaedic surgeon gets a good history of a shoulder injury and x-ray demonstrate no fractures or arthritis, a physical exam is performed. If there is lifting weakness and pain the exam can be repeated after an injection of numbing medicine. If still weak then a MRI or ultrasound of the shoulder should be obtained.
Ultrasound. Mri is an excellent choice. An office based ultrasound by an experienced practitioner is also an excellent test for evaluating the rotator cuff. It is a fraction of the cost of mri, can usually be done during the patient's visit (obviating time and cost for return appointment to review study), and is a wonderful opportunity to educate patients.
Mostly by exam. Rotator cuff disease represents a continuum of symptoms and corresponding pathology. Most people present with pain with specific activities and this is the early course of disease. There is little damage to the rotator cuff and the inflammation is typically resolved with non-operative treatment. Night and/or rest pain represents a likely full thickness rotator cuff tear. Mri confirms diagnosis.