A rule of customary international law is demonstrated by showing evidence of two elements: (1) state practice (objective element); and (2) opinio juris (subjective element).
Digests provide insight into the "view and practice of the Government of the United States in the arena of public and private international law."
Check Sources of State Practice in International Law to locate compiled state papers. Governmental websites, particularly those of foreign ministries, can be useful sources of information on the current international practice of the given country. Additionally, international law yearbooks often contain information on current state practice.