If you are trying to figure out whether or not a U.S. treaty is in force, you would want to look for treaties by topic on the U.S. Department of State website in the most recent version of the pdf document Treaties in Force. Another official source of this information is the Treaties and Other International Acts Series (TIAS) online.
These are finding tools to help you find a treaty on a particular subject. The Thomas treaty search linked to above allows researchers to search for U.S. treaties by subject. The HeinOnline World Treaty Collection linked to above does the same for U.S. & non-U.S. treaties. Both of these databases leverage treaty indexes to enable you to search by subject. The World Treaty Index provided below is not as powerful a search tool as these two databases however, it may contain some materials that are not available through Thomas or HeinOnline and unlike HeinOnline, it is free.