What is the test for establishing that a party received a letter?

In Slater v. Kehoe (1974) 38 Cal.App.3d 819, the court explains the ephemeral nature of the presumption. "If a party proves that a letter was mailed, the trier of fact is required to find that the letter was received in the absence of any believable contrary evidence. However, if the adverse party denies receipt, the presumption is gone from the case. The trier of fact must then weigh the denial of the receipt against the inference of receipt arising from proof of mailing and decide whether or not the letter was received." (Id. at p. 832, fn. 12.)