In NORTH SEA CONTINENTAL SHELF CASES (Germany v Denmark and Netherlands), 1969 ICJ Rep 3, the ICJ articulated the test for OPINIO JURIS as:

1. In North Sea Continental Shelf, the ICJ held (paragraph 77): 'Not only must the acts concerned amount to a settled practice, but they must also be such, or be carried out in such a way, as to be evidence of a belief that this practice is rendered obligatory by the existence of a rule of law requiring it. The need for such a belief, i.e., the existence of a subjective element, is implicit in the very notion of the opinio juris sive necessitatis. The States concerned must therefore feel that they are conforming to what amounts to a legal obligation.' 2. This is the canonical articulation of opinio juris. 3. Hence option B is correct. _Source: UN Charter 1945 / Vienna Convention on Law of Treaties 1969 / ICJ Statute / ARSIWA 2001 — North Sea Continental Shelf Cases, 1969 ICJ Rep 3, para. 77_