Custom is evidence of a general practice accepted as law according to article 38, paragraph 1, of the Statute of the International Court of Justice . Custom is an expression of a claim, demand, or expectation. The two generally accepted constituent elements in customary international law are state practice and opinio juris sive necessitatis or opinio juris [legal opinion of a state]. It is also commonly agreed that custom refers to the practice and opinions of states, and not individuals and other collective entities...
- Doebbler, Curtis F. J.
"This phrase (in full, opinio juris sive necessitates ) connotes an element in the formation of international custom, expressed in art. 38(1)(b) of the I.C.J. Statute as ‘a general practice accepted as law’. ‘It is not sufficient to show that States follow habitually a certain course of conduct, either doing or not doing something. To prove the existence of a rule of international customary law, it is necessary to establish that States act in this way because they recognize a legal obligation to this effect’: Schwarzenberger and Brown, A Manual of International Law (6th ed.), 26." - Grant, John P., et al.
"The two constituent elements in customary international law are state practice and opinio juris sive necessitates , or opinio juris . These elements are expressly stated in article 38, paragraph 1, subparagraph b, of the Statute of the International Court of Justice and have been widely accepted by international judicial bodies. The Court referred to these elements in numerous cases...
Opinio juris is traditionally derived from state’s opinions, but exactly how a state expresses its opinion in relation to an act that is or is not in accordance with the rule alleged to be international law is not always clear. On the one hand, such manifestations of opinion might come from the statements of senior state officials or the official statements of states in an international body such as the United Nations General Assembly. On the other hand, statements of opinio juris might be the result of widely supported resolutions, especially one such as the Declaration on Principles of International Law concerning Friendly Relations and Cooperation among States in Accordance with the Charter of the United Nations (UNGA Res. 2625 (XXV) adopted 24 October 1970), which expressly states it is recognizing principles of international law. Evidence of opinio juris might include diplomatic pronouncements, votes, or other such expression of views by states. Some terms recognized by the International Court of Justice as expressing more than an opinio simpliciter include “ recognition ,” “conviction,” “sense of duty,” and “consciousness,” according to O. Elias, “The Nature of the Subjective Element in Customary International Law,” ICLQ 44, no. 3 (July 1995): 501, 511. It is important that the expression of opinio juris be directed toward an act and concern a sense of legal obligation for it to contribute to forming customary international law." - Doebbler, Curtis F. J.
Article 38 of the International Court of Justice Statute. 108 says that one primary source of international law is custom. Customary international law is made up of the rules of law that come from the consistent actions of states, driven by their belief that the law compels them to act, or to refrain from acting in a certain manner. This sense of obligation is called opinio juris, while state practice refers to the official actions of a government, as seen in a range of possible activities such as official statements during international conferences, diplomatic communications, formal directives to diplomatic agents, decisions made by national courts, legislative actions, or any other measures adopted by governments to address international issues.
This means that you’re looking for two things, first state practice, that is, an action that many states are all doing (or not doing) and second, some kind of legal obligation, opinio juris, that is the reason why they are all doing or not doing the action.
Finding evidence of customary international law can be tricky because the sources are not easy to identify or to find. Moreover, there isn't just one place where all these types of law sources are found. They're spread across different sources like treaties, laws of countries, official government papers, documents produced by intergovernmental organizations and so on. This page gives some basic direction on where to look for these sources of customary international law. It is best to start with secondary sources on your topic and let these publications lead you to the most relevant documents.
Use these resources to find evidence of Customary International Humanitarian Law
Customary International Humanitarian Law by 
Perspectives on the Icrc Study on Customary International Humanitarian Law by 
The Changing Nature of Customary International Law by 
Developments in customary international law : theory and the practice of the International Court of Justice and the international ad hoc criminal tribunals for Rwanda and Yugoslavia by 
The Formation and Identification of Rules of Customary International Law in International Investment Law by 
