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:
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