Final answer:
When determining a law's validity, a judge uses the US Constitution. Key Supreme Court cases such as Schenck v. U.S. and New York Times Co. v. Sullivan provide the tests and standards for what constitutes protected speech under the First Amendment.
Step-by-step explanation:
In the fictional Stolen Valor Act case of US v. Fields, the document a judge would use to determine the law's validity is the US Constitution. The courts have consistently been the key interpreters of the Constitution, ensuring that any law, including those related to free speech, aligns with constitutional principles. In cases such as Schenck v. United States and Brandenburg v. Ohio, the court has ruled on what constitutes protected political speech and what may be considered a limitation on free speech for reasons of public safety or defamation.
In speech-related cases, the courts analyze the intention and content of speech, considering whether it presents a "clear and present danger", creates incitement, or constitutes defamation. This basis is found in landmark cases like New York Times Co. v. Sullivan, which established the standard of "actual malice" for defamation cases involving public officials.