What is the legal test for an accessory to a crime?

"If sufficient evidence is presented at trial to justify the conclusion that a witness is an accomplice, the trial court must so instruct the jury, even in the absence of a request." (People v. Brown (2003) 31 Cal.4th 518, 555.) "[A]n accomplice has a natural incentive to minimize his own guilt before the jury and to enlarge that of his cohorts; accordingly, the law requires an accomplice's testimony be viewed with caution to the extent it incriminates others." (Ibid.)

"An accomplice is . . . defined as one who is liable to prosecution for the identical offense charged against the defendant on trial in the cause in which the testimony of the accomplice is given." (Pen. Code, 1111.) To be charged with the identical offense, the witness must be considered a principal under Penal Code section 31. (People v. Fauber (1992) 2 Cal.4th 792, 833.) Section 31 defines principals to include "[a]ll persons concerned in the commission of a crime . . . whether they directly commit the act constituting the offense, or aid and abet in its commission, or, not being present, have advised and encouraged its commission." An accessory, in contrast to a principal, is not an accomplice. (People v. Fauber, supra, at pp. 833-834.) Penal Code section 32 defines an accessory as "[e]very person who, after a felony has been committed, harbors, conceals or aids a principal in such felony, with the intent that said principal may avoid or escape from arrest, trial, conviction or punishment, having knowledge that said principal has committed such felony or has been charged with such felony or convicted thereof."

The defendant has the burden to prove by a preponderance of the evidence that a witness is an accomplice. (People v. Fauber, supra, 2 Cal.4th at p. 834.) "Whether a person is an accomplice is a question of fact for the jury unless there is no dispute as to either the facts or the inferences to be drawn therefrom." (Ibid.)