What is the test for independently determining the legality of a search warrant?

"We defer to the trial court's express and implied factual findings if supported by substantial evidence, but we independently determine the legality of the search under the Fourth Amendment. [Citation.] Because courts accord a preference to searches and seizures conducted pursuant to a search warrant, 'in a doubtful or marginal case a search under a warrant may be sustainable where without one it would fall.' " (People v. Eubanks (2011) 53 Cal.4th 110, 133.)