Iran routinely gets caught whenever it tries to develop clandestine nuclear facilities and no reason to suspect next time won't be the same
Although the chances of an Iranian sneak-out attempt are relatively great, however, its odds of success are extremely low. With the world’s spy agencies devoting huge resources to tracking events inside Iran, any serious attempt at cheating under a new nuclear deal would probably get caught. If Tehran somehow did manage to cheat without notice, its secret program would nonetheless advance slowly. Moreover, even in the unlikely eventuality of a highly efficient secret effort, Iran would still fall short of a bona fide nuclear weapons arsenal. The major powers, then, are right to focus on getting an agreement that limits Iran’s genuine breakout potential, not its highly questionable sneak-out potential.
Iran has often tried to build advanced nuclear capabilities in secret. But time and again, it has gotten caught in the act. Both of Iran’s uranium enrichment plants, for example, were discovered at early stages of construction. Pessimists point to this past cheating as evidence of Tehran’s untrustworthiness, but that same track record also demonstrates that the United States and its partners cannot be easily duped. Moreover, a diplomatic accord with Iran would not stop Western intelligence agencies from looking out for possible Iranian malfeasance. And the more access the International Atomic Energy Agency’s inspectors have to Iran’s nuclear program, the easier it will be to detect any covert activities.
The agreement is based on verification, not trust. According to the U.S. fact sheet, Iran has agreed to accept the IAEA’s Additional Protocol as well as several even more intrusive monitoring and inspection measures that will provide high confidence that Iran is not making bomb material at its declared nuclear facilities and will significantly strengthen the ability of inspectors to detect clandestine facilities