The Restatement describes the evidence of international law in §103. Of particular interest to the researcher of custom is §103(2)(d) which describes the evidence of pronouncements of states:
(1) Whether a rule has become international law is determined by evidence appropriate to the particular source from which that rule is alleged to derive (§ 102).
(2) In determining whether a rule has become international law, substantial weight is accorded to
(a) judgments and opinions of international judicial and arbitral tribunals;
(b) judgments and opinions of national judicial tribunals;
(c) the writings of scholars;
(d) pronouncements by states that undertake to state a rule of international law, when such pronouncements are not seriously challenged by other states.