What is the test for determining whether a confession was involuntary?

Fifth: in determining whether the deception was of the kind which was reasonably likely to trigger a false confession, we must of necessity perform a task for which we are not particularly well equipped--the determination how another person, whom we know only through the pages of an appellate record, was likely to act under certain stressful circumstances which none of us has ever experienced. Inevitably this exposes one to the charge of being a "parlor shrink." Yet this is precisely the kind of determination which courts are constitutionally bound to make and do make in case after case involving a claim that a confession was involuntary. As the United States Supreme Court pointed out in Haynes v. Washington, supra, 373 U.S. at page 515, 83 S.Ct. at page 1344: "[W]e cannot escape the demands of judging or of making the difficult appraisals inherent in determining whether constitutional rights have been violated."