International Law

According to Restatement of the Law (Third), the Foreign Relations of the United States, section 102(2), "Customary international law results from a general and consistent practice of states followed by them from a sense of legal obligation."

Thus, the elements of customary international law include:

• the widespread repetition by States of similar international acts over time (State practice);
• the requirement that the acts must occur out of a sense of obligation (opinio juris); and
• that the acts are taken by a significant number of States and not rejected by a significant number of States.

When researching customary law, it's useful to start with a good secondary source. 

Occasionally, you will find statements of broad international custom in a general international law treatise.

More often, you will have to look at the evidence of specific nations.  Some nations, including the United States, publish materials that state their practices in given areas, and these are the most authoritative sources for what is a custom.  You may also consider participation in treaties as well as national laws are evidence of a state's customary practices

• United States Practice in International Law volume 1 (1999-2001) and volume 2 (2002-2004) - a thorough survey of U.S. international law practice arranged by topic.
• Restatement of the Law, Fourth, Foreign Relations, available on Westlaw and Lexis.
• Foreign Relations of the United States (FRUS) (1861-1980) available on HeinOnline.

• Yearbooks provide surveys of State practice in international law, often including summaries on case law or legislation - International Yearbooks (HeinOnline).
• —A country’s domestic laws also provide evidence of international custom because they indicate the State’s practice and obligations - See the LRC's Foreign Law guide.