What is the test for establishing the credibility of an employer's credibility?

"In discrimination cases, proof of the employer's reasons for an adverse action often depends on inferences rather than on direct evidence. . . . [E]ven though we may expect a plaintiff to rely on inferences rather than direct evidence to create a factual dispute on the question of motive, a material triable controversy is not established unless the inference is reasonable. And an inference is reasonable if, and only if, it implies the unlawful motive is more likely than defendant's proffered explanation." (Cucuzza v. City of Santa Clara (2002) 104 Cal.App.4th 1031, 1038.) Also " '[s]peculation cannot be regarded as substantial responsive evidence.' [Citation.] In order to raise an issue as to the employer's credibility, the employee must set forth specific facts demonstrating ' "such weaknesses, implausibilities, inconsistencies, incoherences, or contradictions in the employer's proffered legitimate reasons for its action that a reasonable factfinder could rationally find them 'unworthy of credence." ' " (Ibid.; italics omitted.)