What is the test for determining whether a witness in a sexual assault case was an accomplice to the accused?

""[W]henever the testimony given upon the trial is sufficient to warrant the conclusion upon the part of the jury that a witness implicating a defendant was an accomplice, '" the trial court must instruct the jury, sua sponte, to determine whether the witness was an accomplice. [Citation.] If the testimony establishes that the witness was an accomplice as a matter of law, the jury must be so instructed. [Citation.] In either case, the trial court also must instruct the jury, sua sponte, that the testimony of the accomplice witness is to be viewed with distrust [citations], and (2) that the defendant cannot be convicted on the basis of the accomplice's testimony unless it is corroborated...." (People v. Zapien (1993) 4 Cal.4th 929, 982.)