How did president truman guard against communist spies in america?

In 1950, with Senator McCarthy and other arousing fears of Communist spies, Congress passed the Internal Security Act, usually called the McCarran act.
The Internal Security Act of 1950, 64 Stat. 987 (Public Law 81-831), also known as the Subversive Activities Control Act of 1950 or theMcCarran Act, after its principal sponsor Sen. Pat McCarran (D-Nevada), is a United States federal law.
Hope this helps!:)