In 1948, people in the AEC starting thinking that we ought to have a way to detect nuclear tests by other powers. Of the three possible methods of detection discussed (radioactive dust in the atmosphere, acoustic, and seismic) only the first could be quickly implemented, and Oppenheimer was convinced that it couldn't work. He and others at that time didn't believe air bursts would leave dust, the radioactive elements having been reduced to atoms. The Air Force had been given the job of doing it, and found a private company, Tracer Labs, eager to take up the challenge. Oppenheimer was shown to be wrong.
Shortly after the Soviets exploded their first nuclear bomb near Semipalatinsk in September of 1949, the Air Force and Tracer Labs detected the signal, and followed it across the ocean, the US and the UK. Tracer labs was able to pin down the day and nearly the hour of the explosion. Despite independent evidence from intelligence, Secretary of Defense Louis Johnson and later Truman were skeptical.
... Truman was skeptical of the Soviet achievement; he told a senator later that he could not believe “those asiatics” could build so complicated a weapon as an atomic bomb.1706
Rhodes, Richard. Dark Sun: The Making Of The Hydrogen Bomb (p. 373). Simon & Schuster. Kindle Edition.
Ignorant prejudice - it's not just for breakfast.