International Monitoring System
On September 22, 1979, a U.S. satellite detected a nuclear flash near the Prince Edward Islands off Antarctica. Called the Vela Incident, none of the existing nuclear powers took responsibility for it, which troubled the world's powers.
Since then, the world has been working towards a Comprehensive Test Ban Treaty to put an end to the testing of nuclear weapons everywhere. Once all the nuclear powers ratify the treaty, the international Comprehensive Test Ban Treaty Organization (CTBTO) will monitor compliance. The CTBTO has already established a global monitoring facility that can detect nuclear weapons detonations anywhere on the planet.
The map below shows the locations of the current monitoring sites, but note that most markers are not accurately placed. That's deliberate. Apparently, the CTBTO doesn't want you to know the exact locations. In some cases, we've placed the marker closer. After all, if a hostile entity were out to destroy monitoring stations, how difficult would it be to find them? Plus, given the significant redundancy, how many would need to be destroyed to severely degrade the system?
The CTBTO also has a map that shows the locations of its monitoring stations and the locations of past nuclear explosions.
Comprehensive Test Ban Treaty
The Comprehensive Test Ban Treaty, adopted by the United Nations on September 10, 1996 has been signed by 185 countries (including the United States) and 170 countries have ratified it. However, the treaty cannot go into effect until it is ratified by the governments of the nuclear powers. This table shows the nuclear powers and whether they've ratified it. All but India have signed the treaty.
List of parties to the Comprehensive Nuclear-Test-Ban Treaty
What's the holdup on the U.S. Congress ratifying the treaty?
The Brookings Institute helps answer the question: What’s the deal with Senate Republicans and the test ban treaty?
You can get more details about treaty ratification at the Nuclear Threat Initiative website
These videos explain the monitoring system.