What alerted the United States to the Soviet belief that capitalism and Communism could not coexist in the world?

Well, if you have ever read any of Marx's writings it quickly becomes obvious that that was his belief. The Soviet system was built directly on Marx's writings.

If the US was not already alerted by Marx's writings and needed something else, they were willfully ignorant. During the Great Depression many individual Americans became willfully ignorant due to the apparent failure of US capitalism and the USSR's Propaganda (but official US policy remained largely antisoviet as it had been since the Bolshevik revolution). During World War 2 official US policy was to be willfully ignorant, to help keep the USSR as an ally against the Nazis. After the war this willful ignorance gradually became less necessary and it was easier to recognize Stalin's anticapitalistic and antidemocratic activities in the Soviet occupied eastern European zones.