What is the test for cross-examination in the context of the Sixth Amendment?

The "main and essential purpose" of the Sixth Amendment right to confrontation "is to secure for the opponent the opportunity of cross-examination." Delaware v. Van Arsdall, 475 U.S. 673, 678, 106 S.Ct. 1431, 89 L.Ed.2d 674 (1986). However, "[a] defendant's confrontation right ... is not absolute." Dres v. Campoy, 784 F.2d 996, 998 (9th Cir.1986). The prior testimony of a witness is admissible at trial if it meets a two-prong test. First, the prosecutor must prove that the witness is unavailable to testify at trial. Second, the defendant must have had the opportunity to cross-examine the witness at the prior hearing. See id. at 999.