Article 38(1) of the Statute of the International Court of Justice tells the court to decide cases before it by applying
"(a) international conventions, ....
(b) international custom, as evidence of a general practice accepted as law;
(c) the general principles of law recognised by civilised nations
(d).... judicial decisions and the teachings of the most highly qualified ... as subsidiary means for the determination of the rules of law."
International custom requires both state practice and proof of opinio juris sive necessitatis ie that the state had felt legally obliged to act in that way.
To find commentary on SOLO use subject search
Customary law, International
Vienna Convention on the Law of Treaties (1969)
art. 53: A treaty is void if, at the time of its conclusion, it conflicts with a peremptory norm of general international law. For the purposes of the present Convention, a peremptory norm of general international law is a norm accepted and recognized by the international community of States as a whole as a norm from which no derogation is permitted and which can be modified only by a subsequent norm of general international law having the same character.
art. 66 (a) disputes on such a norm's existence can be submitted to ICJ.
An official, internationally agreed list of norms does not (as of Jan 2017) exist but it would - arguably - include both positive eg right to peoples' self determination & negative eg prohibition of genocide
" a concept embodying the community interest and reinforced by its link with public morality [existing] in modern international law as a matter of necessity” ( Peremptory Norms in International Law by Alexander Orakhelashvili at p. 577. See left hand column for this work).
Jus cogens (International law) is a specific subject search term to try in SOLO