What is the test for determining whether a statement was illegally obtained?

On appeal, we "must accept the trial court's resolution of disputed facts and inferences, and its evaluations of credibility, if they are substantially supported. [Citations.] However, we must independently determine from the undisputed facts, and those properly found by the trial court, whether the challenged statement was illegally obtained. [Citation.]' " (People v. Johnson (1993) 6 Cal.4th 1, 25.)