What is the test for a purported confession in a criminal case?

Clearly, a purported confession by an accused to any crimes that are charged "tends to establish the defendant's commission of the crime." (See id., at p. 753, 312 P.2d at p. 675.) " 'If the defendant had confessed, proof of the confession was a part of the case of the People and it was the duty of the [27 Cal.3d 331] District Attorney to offer it before resting his case, when the testimony was then available and there was no reason for not offering it in chief.' " (People v. Robinson (1960) 179 Cal.App.2d 624, 630, 4 Cal.Rptr. 50, 54.)