"There are expressions in the decisions to the effect that the separate character of property acquired after marriage [202 Cal. App. 2d 91] is to be established by 'clear and convincing evidence,' 'clear and decisive proof,' 'clear and satisfactory proof.' [Citing cases.] These expressions state a rule of evidence directed to the trial court; and if that court finds that the evidence meets the rule, a reviewing court must accept that determination as conclusive if there is substantial evidence to support it. [Citations.] ... Whether the evidence adduced to overcome the presumption of community property is sufficient for the purpose is a question of fact for the trial court. [Citing cases.]"