During the Cold War with the Soviet Union and the arms race that accompanied it, the United States periodically engaged in arms control negotiations and signed off to several arms control agreements.
One of the challenges regarding enforcement of those agreements was the ability to verify that the Soviet Union actually intended to abide by their terms. Uncertainty always remained about whether or not the Soviet Union continued to manufacture and deploy nuclear missiles and whether or not the United States possessed the means to detect such violations.
The same challenges face the United States today regarding the new agreement with Iran about their own nuclear program.
Fortunately, the State Department released this video explaining the strategy by which we will confirm that Iran abides by the terms of the new agreement: