The UN and COE offer up-to-date status information, but other intergovernmental organizations (e.g., OAS, AU, ASEAN, ECOWAS, SADC) do not always have up-to-date information.
If status information is crucial, contact the IGO, or the embassy or foreign ministry of the country you're interested in.
Signatories, Ratifications, Status, Reservations
MNCAT subject headings: treaties ratification or treaties reservations.
This can be the most difficult information to locate for many treaties. The best place to start is with online sources. You may also need to use the treaty indexes such as Treaties in Force, Multilateral Treaties Deposited with the Secretary General, or Multilateral Treaties: Index and Current Status.
For the most up-to-date information, call the U.S. State Department's Treaty Affairs Office (has separate numbers for U.S. treaties, U.N. treaties, and foreign/non-U.S. treaties), the United Nations treaty information office (has separate numbers for bilateral and multilateral treaties); phone numbers can be obtained from the web, or the Reference Office of the Law Library.
The web is often the best source for status information. Some of the useful sites are listed below. Again, this is only a sampling of what is out there on the web.
Environmental Treaties and Resource Indicators
Select Documents on International Affairs Series
Hague Conventions on Private International Law (Status List)
BASE PACTE - Traités et accords de la France (in French, but provides citations to U.N.T.S. and other sources)