What is the test for determining the credibility of a witness in a sexual assault case?

We approach this factual inquiry mindful that a trial court's Penal Code section 1538.5 ruling involves a two step process. In the first step, a trial court makes factual findings to which we must give proper deference. "[T]he power to judge the credibility of the witnesses, resolve any conflicts in the testimony, weigh the evidence and draw factual inferences, is vested in the trial court. On appeal all presumptions favor the exercise of that power, and the trial court's findings on such matters, whether express or implied, must be upheld if they are supported by substantial evidence." (People v. Leyba (1981) 29 Cal.3d 591, 596-597, 174 Cal.Rptr. 867, 629 P.2d 961.) It is only with reference to the second step, i.e., the legal effect of the factual findings that we can exercise our independent judgment. (Id., at p. 597, 174 Cal.Rptr. 867, 629 P.2d 961.)