Here are some ways to find out which countries have signed what kind of treaties. Also, bear in mind that membership in international organizations de facto carries some kinds of agreements and cooperation (i.e. European Union).
on the internet:
UN Database of Treaties – All treaties entered into by any member state.
CIA World Factbook – see which international organizations a country belongs to. Choose your country and click on government to the bottom of the screen.
Treaties in Force – Treaties America has signed.
Search Nexis Uni for the full text of lapsed and current U.S. Treaties.
In Nexis Uni, go to 1) advanced search 2) search by content type then 3) treaties under Statutes and Legislation