International custom is derived from the historical actions of nations or state practice, as it is known.
"[C]ustomary international law results from a general and consistent practice of states followed by them from a sense of legal obligation." Restatement of the Law Third, Foreign Relations of the United States § 102(2) (1987) (available on HeinOnline; in print at Law Reserve & Law Annex Ref [KF 4651 R47 1987]).
Establishing international custom is perhaps the most difficult task one can undertake in international law research. As a first step, start by gaining a good understanding of the concept of customary law. Guides can be useful for this task.
Step 1: Look at Secondary Sources to determine if someone has already researched this area. Secondary sources include Books, Articles, Encyclopedias and Reports.
Step 2: Look at non-State sources of state practice, like IGO resolutions or actions by states in other IGOs. Also look at IGO practice and the decisions of international tribunals.
In other words, first you must determine the general and uniform state practice, and then you must analyze whether the states considered themselves legally bound to behave that way. Determining state practice is not a simple process, nor one that anyone other than a scholar is generally able (or willing) to undertake.
Mr. Anthony D’Amato
Judd and Mary Morris Leighton Professor of Law
Northwestern University School of Law
In addition to international conventions, the Statute of the International Court of Justice sets out three other sources of international law in Art. 38:
1. international custom, as evidence of a general practice accepted as law;
2. the general principles of law recognized by civilized nations;
3. judicial decisions and the teachings of the most highly qualified publicists of the various nations, as subsidiary means for the determination of rules of law
Determining the general principles which underlie the law of all nations is a task of comparative international law. There are several tools in this area, but again, it is one where using the existing work of scholars (as opposed to trying to figure it out for yourself) is probably the best way to go. The other is to look at all national systems. See the Foreign Law guide.
The subsidiary means of determining international law is through the decisions of international and national tribunals and through the writings of jurists. Many of these resources have been mentioned above because they are both evidence of customary international law and a subsidiary means of establishing customary international law.
Judge A. A. Cançado Trindade
International Court of Justice
Inter-American Court of Human Rights