What is the test for determining whether a defamatory statement constitutes a fact or opinion?

In recognizing that the "critical determination of whether the alleged defamatory statement constitutes fact or opinion is a question of law," the Okun court observes that "[t]he distinction frequently is a difficult one," and goes on to elaborate that "what constitutes a statement of fact in one context may be treated as a statement of opinion in another, in light of the nature and content of the communication taken as a whole." (Okun v. Superior Court, supra, 29 Cal.3d at p. 450, 175 Cal.Rptr. 157, 629 P.2d 1369.)