To physiodawn, when attempting a differential diagnosis , where the possibility of referred pain etc exists ( as would be true for most musculoskeletal pain) tests can be useful. Such as neural tension tests, dural stretches generally, active movements ,assessments of tight structures etc. What must be borne in mind however is that no test is 100 percent accurate, certainly not the slump or its brachial equivalent. False negatives will lead to innapropriate treatment. I have found that it is better to assume there is referred pain and other altered sensations unless proven otherwise, especially in the light of tenderness elicited at the relevant Wikipedia reference-linkfacet joint with mobs.
I suggest two methods to be more certain of joint involvement. "Traction rotations "performed with patient sitting , where therapist grips the head using forearm hand and chest, while exerting traction and rotating the head into the direction of the percieved referred pain. This will in most cases ,where cervical joints are involved, relieve some of this pain immediately. try 3 by 20 seconds moving the head back and forth into and out of rotation.
AP mobs are also very good to reveal referred pain , if not down to the elbow then along the C5 and C6 dermatome. Continuous AP mobs through a range that just causes discomfort ( but not necessarily the referred pain) for up to ten minutes will confirm the diagnosis by inference, when referred pain is relieved.
All the best