The just-released Treaties in Force 2009 publication by the U.S. Department of State will tell you what bilateral and mulitlateral treaties to which the United States is a party are currently in force.  This is an annual publication also available in print and on HeinOnline (subscription database available to Yale community only).

Treaty research can be complicated so there are plenty of research guides and databases to help with your research.  You will find them on our Foreign and International Research Resources page.