What is the test for making an offer of proof to the trial court that the complaint can be amended to state a viable cause of action?

Stated another way, no offer of proof was made to the trial court that the complaint could be amended to state a viable cause of action. There is authority that the issue of leave to amend is always open on appeal. (See Code Civ. Proc., 472c, subd. (a); City of Stockton v. Superior Court (2007) 42 Cal.4th 730, 746.) "'While such a showing can be made for the first time to the reviewing court [citation], it must be made.'" (Medina v. Safe-Guard Products, Internat., Inc. (2008) 164 Cal.App.4th 105, 112-113, fn. 8.)