What is the test for improperly obtained evidence in the context of the Fourth Amendment?

"The Fourth Amendment protects '[t]he right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures,' but 'contains no provision expressly precluding the use of evidence obtained in violation of its commands,' [citation]. Nonetheless, our decisions establish an exclusionary rule that, when applicable, forbids the use of improperly obtained evidence at trial. [Citations.]" (Herring v. United States (2009) 555 U.S.135, 699] (Herring).)