What is the test for determining whether a search warrant has to be issued to search a vehicle that has been stopped and searched by police?

In the relatively early case of Brinegar v. United States, 338 U.S. 160, 69 S.Ct. 1302, 93 L.Ed. 1879, the majority of the court, on the basis of their assumption that the officers there intended to search the car from the outset and had adequate facts prior to stopping it to warrant such a search, were not required to discuss the question whether the officers would have been justified in stopping the car on the basis of some lesser quantum of evidence for purposes of interrogating the driver, but with no right to conduct a search thereof.