What is the test for determining whether an assessment officer has properly performed their duties and properly applied a valid method of valuation?

"The law presumes that assessment officers have properly performed their duties and consequently that assessments are regularly and correctly made. [Citations.] When a taxpayer-plaintiff claims that the assessor and the county board have erroneously applied a valid method of property valuation, the courts will overturn the decision only when no substantial evidence supports it; when the plaintiff challenges the validity of the valuation method itself, the courts are faced with a question of law. [Citations.]" (Dressler v. County of Alpine (1976) 64 Cal.App.3d 557, 566, 134 Cal.Rptr. 554.)