Customary International Law defined
Brownlie in his Principles of Public International Law, “the number of parties, the explicit acceptance of rules of law, and, in some cases, the declaratory nature of the provisions produce a strong law-creating effect at least as great as the general practice considered sufficient to create a customary rule.”
the authors of the Public International Law in a Nutshell list the elements of customary international law to be the following:
The "practice of states" means official government conduct reflected in a variety of acts, including official statements at international conferences and in diplomatic exchanges, formal instruction to diplomatic agents, national court decisions, legislative measures or other acts taken by governments to deal with matters of international concern."
Buergenthal and Murphy. Public International Law in a Nutshell, pp 22 – 23.
Another concept appears in the area of customary law, peremptory norms(jus cogens), which are rules and principles considered binding by the international community. These are generally thought to include prohibitions on the use of force, crimes against humanity, war crimes, piracy, genocide, and slavery.
Orakhellashvili, Alexander. Peremptory Norms in International Law. Oxford & New York: Oxford University Press, 2006
Finding State Practice
Digests of United States Practice of International Law
These digests contain documents of the presidents and the secretaries of state as they relate to international law. As such they are evidence of customary law.
Whiteman, Digest of International Law. Covers 1940-60.
Hackworth, Digest of International Law. Covers 1906-39.
Moore, A Digest of International Law. Covers 1776-1906.
Wharton, A Digest of the International Law of the United States, 2d ed. 1887 Also in HeinOnline
Murphy, Sean D. United States Practice in International Law. Cambridge, UK ; New York, NY : Cambridge University Press, 2002. 2 vols. v. 1. 1999-2001 -- v. 2. 2002-2004.
American Law Institute. Restatement of the Law, Third, Foreign Relations Law of the United States: As Adopted and Promulgated. St. Paul Minn: American Law Institute, 1987.
" the foreign relations law of the United States consists not only of international law but also of selected domestic law of the United States. The sources of international law on which this Restatement is based include foreign as well as United States sources. The United States domestic law that is part of the foreign relations law of the United States includes notably the Constitution, with its provisions for a judiciary that has substantial independent authority in formulation of law, the separation of powers between the legislative, executive, and judicial branches, the special constitutional role of the Senate with respect to treaties, and the federal structure of government. All of these significantly affect the foreign relations law of the United States." from the Forward to Restatement (Third) of Foreign Relations Law Foreword (1987)
Sources of State Practice in International Law
Gaebler, Ralph, and Alison A. Shea. eds. Sources of State Practice in International Law. Leiden, NL & Boston: Brill, 2014
"Sources of State Practice in International Law is a descriptive bibliography of both electronic and printed sources of information containing the text of treaties and the record of diplomatic activity of important jurisdictions around the world. As such, it includes an up-to-date description of national treaty portals and other valuable Internet-based sources. At the same time, it also includes descriptions of printed sources providing access to treaties and official diplomatic documentation difficult to locate in standard compilations. In addition, this work includes a narrative section for each jurisdiction summarizing issues related to treaty succession and treaty implementation in municipal law."