What is the test for disclosing an informant in a criminal case?

In order for a defendant to succeed in a motion to disclose an informant, the defendant must demonstrate "a reasonable possibility that the informer could give evidence on the issue of guilt that might result in defendant's exoneration. This rule of evidence is founded on the principle that nondisclosure in this situation would result in a denial of a fair trial to defendant." (People v. Tolliver (1975) 53 Cal.App.3d 1036, 1043, 125 Cal.Rptr. 905.)