What is the test for a judge at the Court of Appeal to determine whether or not to retry the issue of custody on appeal?

governed by well-established principles: "It has been held repeatedly that the trial judge having heard the evidence, observed the witnesses, their demeanor, attitude, candor or lack of candor, is best qualified to pass upon and determine the factual issues presented by their testimony; and, . . . 'this is especially true where the custody of minor children is involved. An appellate tribunal is not authorized to retry the issue of custody, nor to substitute its judgment for that of the duly constituted arbiter of the facts.' It is the trial court's responsibility to pass on the credibility of witnesses, the weight to which their testimony is entitled, and the inferences to be drawn from the evidence. On appeal it is, of course, the duty of this court to view the evidence and the reasonable inferences to be drawn therefrom in the light most favorable to the successful party in the court below." (Ducharme v. Ducharme (1957) 152 Cal.App.2d 189, 193; see Sanchez v. Sanchez (1961) 55 Cal.2d 118, 126 ["There were, as is not unusual in a proceeding of this nature, conflicts both as to probative facts and as to the proper inferences to be drawn from such facts. But, as is true in all appellate reviews, and most emphatically in this type of controversy, it is not the function of this court to reweigh conflicting evidence and redetermine findings; neither is this court vested with discretion to be exercised in the premises. Our function has been fully performed when we find in the record substantial evidence which supports the essential findings of the trial court."]; see also In re Marriage of Fajota, supra, 230 Cal.App.4th at p. 1497.)