What is the test for establishing that privilege is a privilege?

" . . . (T)he recognition of a privilege does not mean that it is without conditions or exceptions. The social policy that will prevail in many situations may run foul in others of a different social policy, competing for supremacy. It is then the function of a court to mediate between them, assigning, so far as possible, a proper value to each, and summoning to its aid all the distinctions and analogies that are the tools of the judicial process." Clark v. United States, 289 U.S. 1, 13, 53 S.Ct. 465, 469, 77 L.Ed. 993 (Cardozo, J.).