I wonder if carbon dating would work. Seems like it ought to, since C14 is produced in the upper atmosphere, so if's been sealed then it wouldn't get any new C14. The small amount of decay over 42 years might be trouble to measure.

A potentially crazier idea: measure the trace amounts of uranium, plutonium, and other long-lived fallout products. Atmospheric nuclear testing continued on a semi-regular basis until 1980, and so the makeup and concentration of the fallout would be different if it had been unsealed after 1972.