Scalene muscle block is often performed to assist with the clinical differentiation of primary sources of pain and weakness in the upper limb when the differential diagnosis includes thoracic outlet syndrome. This presentation offers a simple clinical method to assess needle placement in the scalene muscle before an injection of local anesthetic which, if properly placed, weakens the scalene muscle and often leads to temporary relief of symptoms associated with neurovascular compression. An appropriate scalene block response provides assistance with medical decision making.