In international law, opinio juris requires evidence which usually comes from resolutions. For example, general assembly resolutions. However, just because there is a general assembly resolution, does not mean that the resolution is law itself. There has to be state practice connected to the resolution. In international law general assembly resolutions must be approached with caution.
Ex. Nicaragua case: court looks at issue of opinio juris. Is there a belief on the part of states that they are interfering with other states because they feel they have the right to do so? The court concluded there was not enough evidence for opinion juris; the USA was phrasing their interference in Nicaragua as an exception to customary state law. Their interference was for extra-legal policy reasons; it was not a matter of law.