The U.S. Supreme Court employs several tests to determine whether something violates the Establishment Clause. The most prominent is the so-called “Lemon test” from the Court’s 1971 decision Lemon v. Kurtzman. The Lemon test requires that the government have a secular purpose, that government regulation does not have a primary effect of advancing or inhibiting religion, and that it does not create an excessive entanglement between church and state.

Justice Sandra Day O’Connor introduced another test, which she called a “refinement” of the Lemon test. Her endorsement test asks whether a reasonable observer, familiar with the underlying circumstances, reasonably would believe that the government is endorsing religion.