What is the test for using an undercover employee as an informer in a criminal case?

'This is a case where the Government's informer was the sole participant, other than the accused, in the transaction charged. The informer was the only witness in a position to amplify or contradict the testimony of government witnesses. Moreover, a government witness testified that Doe denied knowing petitioner or ever having seen him before. We conclude that, under these circumstances, the trial court committed prejudicial error in permitting the Government to withhold the identity of its undercover employee in the face of repeated demands by the accused for his disclosure.' (Roviaro v. United States, supra, 353 U.S. 53, 64--65, 77 S.Ct. 623, 630.)