What is the test for establishing that an agreement is void ab initio?

The rule, however, has no relevance and should not be applied in this case. First, since the requirement here involves an unlawful agreement which is contrary to public policy and unlawful, the agreement is void ab initio (see City of Los Angeles v. Morgan (1951) 105 Cal.App.2d 726, 733, 234 P.2d 319) and would entail requiring the court to uphold an unlawful agreement, a result disfavored.