What is the test for establishing an agency as a matter of law?

"A contention that the evidence establishes a particular fact as a matter of law when the fact finder has determined the fact to the contrary is but another way of asserting that the evidence is insufficient to support the determination of the fact finder." (Wickham v. Southland Corp. (1985) 168 Cal.App.3d 49, 54, 213 Cal.Rptr. 825.) The existence of an agency is a factual question within the province of the trier of fact whose determination may not be disturbed on appeal if supported by substantial evidence. (Id. at p. 55, 213 Cal.Rptr. 825.)