Earlier quoted context omitted.
You could just look for isotopes from the atmospheric nuclear tests that were conducted? Doesn't need to be a reactor I would assume?
Indeed. That's one of the reasons why the "Before Present" time scale (that uses 1/1/1950 as the epoch) is sometimes called "Before Physics" (another one being that 1950 is less and less "present", of course)!
I guess we can say this is when humankind reached adulthood, became able to drink and got a driver’s license. And a gun.