Some excellent online treaty research guides are available from various law school libraries with large international collections: e.g., Boston University, Columbia, Georgetown, N.Y.U. The Hein Online Treaties and Agreements Library links to some of them. Use search terms such as <treaty research>
For details about U.S. Treaty Practice, see:
Robert E. Dalton, Chapter 20, "National Treaty Law and Practice: United States," National Treaty Law and Practice, eds. Hollis et al (2005).