What is the test for a waiver of a defendant's right to counsel during a custodial interrogation?

"[Once] an accused has invoked his right to have counsel present during custodial interrogation, a valid waiver of that right cannot be established by showing only that he responded to further police-initiated custodial interrogation even if he has been advised of his rights.... [A]n accused, ... having expressed his desire to deal with the police only through counsel, is not subject to further interrogation by the authorities until counsel has been made available to him, unless the accused himself initiates further [56 Cal.App.4th 581] communication, exchanges, or conversations with the police." (Edwards v. Arizona, supra, 451 U.S. at pp. 484-485, 101 S.Ct. at pp. 1884-1885, fn. omitted.)