Jerram,
My fault about scanned area, I read 'proximal biceps region to the mid arm', not forearm as is correct. About biceps, forearm supination is performed by the radial attachment and the muscle as a whole, not long or short head. Therefore, there won't be any difference in pain intensity at the elbow if the problem is at the shoulder. There will be a difference at the shoulder though. So, assuming that the long head is involved and not the short, testing of supination won't clarify which head is injured. Testing of shoulder will do. I hope this was what you asked...
ilias