What is the test for a showing of prejudice when a defendant is denied counsel throughout the criminal proceedings?

The rule we adopted in MontesLopez is based in part on the practical difficulties one would face in trying to prove that the outcome of the merits hearing would have been different had counsel been able to assist. Id. at 1092. In most cases it would be next to impossible to determine what the evidentiary record would have looked like had counsel been present. The same practical difficulties explain why, in the Sixth Amendment context, a showing of prejudice is not required when the defendant is denied counsel (or wrongly denied counsel of choice) throughout the entire criminal proceeding. See United States v. GonzalezLopez , 548 U.S. 140, 14850, 126 S.Ct. 2557, 165 L.Ed.2d 409 (2006). In that context, too, it would usually be impossible to determine what different decisions counsel (or counsel of choice) might have made, and what impact those decisions might have had on the outcome of the proceeding. Id. at 15051, 126 S.Ct. 2557. Given "the difficulty of assessing the effect of the error," id. at 149 n.4, 126 S.Ct. 2557, automatic reversal is required.