Researching customary international law is perhaps the most laborious type of international law research. The most rigorous approach would be finding real world evidence of state practice, and then separately finding evidence of opinio juris supporting that practice. Most frequently, however, research consult secondary sources, which may argue that certain customary rules exist. The best secondary sources will contain detailed information about the primary sources from which the rules were derived, allowing the researcher to work backwards to build a case that a certain rule of customary law in fact exists.
The following resources are some of the helpful starting places when formulating research on customary law:
This online encyclopedia has a very detailed article and research guide on understanding and researching customary law. Max Planck is available through the law school. You can access it at the law school directly by clicking the link above on any law school computer or from home by logging into the Law Library's electronic resources.
This book provides an in-depth explanation of the origins of customary law, as well as a modern discussion of the ways in which the different sources of international law interact.
This book takes an interdisciplinary approach to the origins and issues associated with determining customary international law. It delves into the interplay between politics, diplomacy, and ethics in determining state practice and opinio juris.
Chapter III of this book provides great insight into the sources of customary international law and the distinctions between state practice and official conventions.