What is the test for confronting a witness?

2 As stated in California v. Green, 399 U.S. 149, 158, 90 S.Ct. 1930, 1935, 26 L.Ed.2d 489, 'Confrontation: (1) insures that the witness will give his statements under oath--thus impressing him with the seriousness of the matter and guarding against the lie by the possibility of a penalty for perjury; (2) forces the witness to submit to cross-examination, the 'greatest legal engine ever invented for the discovery of truth' (fn. omitted); (3) permits the jury that is to decide the defendant's fate to observe the demeanor of the witness in making his statement, thus aiding the jury in assessing his credibility.'