What is the test for probable cause in the context of searches under the Fourth Amendment?

"In cases in which the Fourth Amendment requires that a warrant to search be obtained, 'probable cause' is the standard by which a particular decision to search is tested against the constitutional mandate of reasonableness." (Camara v. Municipal Court, supra, 387 U.S. 523, 534, 87 S.Ct. 1727, 1733.) "[P]robable cause is a fluid concept--turning on the assessment of probabilities in particular factual contexts--not readily, or even usefully, reduced to a neat set of legal rules." (Illinois v. Gates (1983) 462 U.S. 213, 232, 103 S.Ct. 2317, 2329, 76 L.Ed.2d 527.) Probable cause is a " 'practical, nontechnical conception' " dealing with " 'factual and practical considerations of everyday life on which reasonable and prudent men, not legal technicians, act.' " (Id. at 231, 103 S.Ct. at 2328.) "Unfortunately, there can be no ready test for determining reasonableness other than by balancing the need to search against the invasion which the search entails." (Camara v. Municipal Court, supra, 387 U.S. at pp. 536-537, 87 S.Ct. at pp. 1734-1735.)