Researchers can start with the convenient list maintained by Mary Rumsey (University of Minnesota Law Library): Frequently-Cited Treaties and Other International Instruments.
A useful trick is to search on the name of the agreement in the law review databases of Westlaw or Lexis. There will be considerable variation, but it is usually not difficult to pick out the most accurate citation, which of course will need to be verified.
If it is necessary to construct a citation from scratch, the researcher will need to refer to the rule in either The Bluebook (19th ed. 2010) or the ALWD Citation Manual (2d ed. 2003). In the former, it is Rule 21.4; in the latter, Rule 21.
As to Rule 21 in the ALWD Citation Manual, it must be said that it is incomplete, inadequate, and inaccurate. A single example will have to suffice: In chart 21.1 accompanying the rule (found only on the web, not in the book), U.N.T.S. is given last in a list of 17 authorized sources, and, to compound matters, it is described as "unofficial"! In short, international agreements should not be cited according to the ALWD Citation Manual until the rule is revised and improved.
Rule 21.4 of The Bluebook has several sub-rules. The critical one is 21.4.5 on treaty sources. Condensing sub-rule 21.4.5 to its essentials produces the following:
Finally, table T.4 on p. 428-29 gives the titles, abbreviations, and dates of coverage of 18 authorized citation sources.
Use these links to access Tarlton's other Foreign & International Law research guides.