What is the test for determining whether a confession was voluntary or coercive?

"We review independently a trial court's determinations as to whether coercive police activity was present and whether the statement was voluntary. [Citation.] We review the trial court's findings as to the circumstances surrounding the confession, including the characteristics of the accused and the details of the interrogation, for substantial evidence. [Citation.] '[T]o the extent the facts conflict, we accept the version favorable to the People if supported by substantial evidence.' [Citation.]" (People v. Guerra, supra, 37 Cal.4th at p. 1093.)