What is the test for obtaining consent for a warrantless search?

As the United States Supreme Court explains, "when the prosecution seeks to justify a warrantless search by proof of voluntary consent, it is not limited to proof that consent was given by the defendant, but may show that permission to search was obtained from a third party who possessed common authority over or other sufficient relationship to the premises or effects sought to be inspected." (United States v. Matlock, supra, 415 U.S. at p. 171, 94 S.Ct. 988, fn. omitted; see Illinois v. Rodriguez (1990) 497 U.S. 177, 188-189, 110 S.Ct. 2793, 111 L.Ed.2d 148.) The "common authority" theory of consent rests "on mutual use of the property by persons generally having joint access or control for most purposes, so that it is reasonable to recognize that any of the co-inhabitants has the right to permit the inspection in his own right and that the others have assumed the risk that one of their number might permit the common area to be searched." (United States v. Matlock, supra, 415 U.S. at p. 171, fn. 7, 94 S.Ct. 988; People v. Haskett, supra, 30 Cal.3d at p. 856, 180 Cal.Rptr. 640, 640 P.2d 776.)