What responsibility does a police officer have in determining whether a suspect waives a substantial right?

'Heretofore the courts have thought that 'the serious and weighty responsibility' of making a determination as to whether there was a waiver of a substantial right was lodged in the trial judge. (Johnson v. Zerbst, 304 U.S. 458, 465, 58 S.Ct. 1019, 82 L.Ed. 1461.) Now, apparently, the policeman is supposed to make the determination as to whether and when the suspect makes an intelligent and knowledgeable waiver. The simple stating of the proposition makes it clear that the successful interrogation of suspects will be next to impossible under any such rule.