It's very difficult to give a comprehensive answer to this question, because it's safe to assume that the United States still has a very large set of Cold War secrets that remain classified.

The United States attempted to place a large number of agents and provocateurs on the ground in the Soviet Union after World War II (and in China after the collapse of the Nationalist regime) without much success, thanks to the Soviet agent Kim Philby. Philby and other Soviet infiltrators did grievous damage to U.S. intelligence efforts. (This was a major factor in the U.S. decision to rehabilitate former Nazi officials and German military officers who had fought on the Eastern Front, in order to gain access to their network of analysts and spies.)

The frustration with the failure of human intelligence efforts led the United States to develop a vast infrastructure for monitoring the Soviet Union through technical means. Spy planes and satellites photographed the Sovet Union from above. Surveillance planes and fighters would simulate an attack run on Soviet cities just to trigger an air defense response so it could be analyzed. U.S. submarines prowled Soviet waters and tapped their "secure" underwater communications cables. In 1974, the U.S. covert Project Azorian salvaged part of the Soviet submarine K-129, which had been lost several years before.

U.S. aerial intelligence scored a number of significant coups throughout the Cold War. It was this reconnaisance that revealed there was no missile gap. U-2 spyplanes discovered the Soviet buildup in Cuba and gave Kennedy a chance to stop the Soviet Union from establishing a nuclear base which would have radically altered the balance of power. Aerial recon prevented the U.S. from bombing Chinese or Soviet positions in North Vietnam which could have widened the war.