Samples can be retrieved from the atmosphere after an accident (Chernobyl) or tests (North Korea, Pakistan and India) and used for later reference.
You are correct.
:-)
In 1949, Oppenheimer knew the Soviets were getting close to the atomic bomb, so he had military aircraft in Japan daily sample the winds blowing east from Russia.
As a result, the US knew of the first Russian nuke test before it was publicly announced by them a few days later.