I am often confronted by "hard to diagnose and persistent shoulder pain" that is incorrectly labeled "rotator cuff syndrome". The long head of the biceps is susceptible to the aforementioned "hard to diagnose, difficult to treat" tendinitis that once correctly identified can in fact be quite easily treated.
The key is to accurately locate the source of the pain as the biceps tendon long head in the bicipital groove, and then to make it geographically available so that treatment is accurately delivered. In the diagram below, you can see the tendon groove and the long head tendon that lives there.
In normal standing posture, the biceps groove lives toward the inside of the humerus closer to your body so that when your arm is against your body and your hand in front of you or at your side naturally, the biceps groove is "hidden". In this case, palpation of the groove is impossible, and direct treatment is ineffective.
In order to palpate the biceps groove, externally rotate your arm, and you will feel a little bump under your fingers as the edge of the groove goes by. If your shoulder hurts under your fingers as the groove goes by, you can safely assume that your biceps tendon is involved.
In order to effectively treat the long head of the biceps tendon, you need to expose the biceps groove by externally rotating the arm so that the biceps groove faces forward. Once positioned as described, you should perform ice massage for about 5 or 10 minutes over the biceps groove and humeral head, and in most cases that will make all the difference.