What is the test for judicial review of legislative motives?

[32 Cal.App.4th 1624] County of Los Angeles also quotes Soon Hing v. Crowley (1885) 113 U.S. 703, 5 S.Ct. 730, 28 L.Ed. 1145 on the rule restricting judicial review of legislators' motives. "[T]he rule is general with reference to the enactments of all legislative bodies that the courts cannot inquire into the motives of the legislators in passing them, except as they may be disclosed on the face of the acts, or inferrible from their operation, considered with reference to the condition of the country and existing legislation. The motives of the legislators, considered as the purposes they had in view, will always be presumed to be to accomplish that which follows as the natural and reasonable effect of their enactments. Their motives, considered as the moral inducements for their votes, will vary with the different members of the legislative body. The diverse character of such motives, and the impossibility of penetrating into the hearts of men and ascertaining the truth, precludes all such inquiries as impracticable and futile." (Id. at pp. 710-711, 5 S.Ct. at p. 734.)