What is the test for determining the credibility of witnesses?

We note initially that we must review these proceedings as full hearings on the issues by the trial court acting as finder of fact: "The power to judge credibility of witnesses, resolve conflicts in testimony, weigh evidence and draw factual inferences, is vested in the trial court. On appeal all presumptions favor proper exercise of that power, and the trial court's findings--whether express or implied--must be upheld if supported by substantial evidence." (People v. Superior Court (Keithley) (1975) 13 Cal.3d 406, 410, 118 Cal.Rptr. 617, 530 P.2d 585.)