What is the legal test for probable cause based on an informant's tip?

Probable cause based on an informant's tip is evaluated under a "totality-of-the-circumstances" test. (Illinois v. Gates (1983) 462 U.S. 213, 230, 103 S.Ct. 2317, 2328, 76 L.Ed.2d 527; People v. Medina (1985) 165 Cal.App.3d 11, 17, 211 Cal.Rptr. 216.) Where an "unquestionably honest citizen comes forward with a report of criminal activity--which if fabricated would subject him to criminal liability-- ... rigorous scrutiny of the basis of his knowledge [is] unnecessary. [Citation.] Conversely, even if ... some doubt [exists] as to an informant's motives, his explicit and detailed description of alleged wrongdoing, along with a statement that the event was observed firsthand, entitles his tip to greater weight than might otherwise be the case." (Id. 462 U.S. at pp. 233-234, 103 S.Ct. at p. 2330.)