What is the test for determining whether a Fourth Amendment violation has occurred?

"Whether a Fourth Amendment violation has occurred 'turns on an objective assessment of the officer's actions in light of the facts and circumstances confronting him at the time' [citation], and not on the officer's actual state of mind at the time the challenged action was taken." (Maryland v. Macon (1985) 472 U.S. 463, 470-471, 105 S.Ct. 2778, 2782-2783, 86 L.Ed.2d 370; quoting Scott v. United States, supra, 436 U.S. at p. 136, 98 S.Ct. at p. 1723.)