It's usually pretty easy to figure out who were the Communists prior to Pearl Harbor. They're the ones who were anti-Nazi until the August 1939 Nazi/Soviet non-aggression pact, then became very anti-British, then, suddenly, around June 22, 1941 became very anti-Nazi again.
If they were consistently opposed to the Nazis throughout that period, especially during the ostensible Nazi/Soviet alliance, then they may have been left-wing, but they weren't Communist suck-ups.