What is the test for impeaching a defendant's credibility in a criminal case?

"If a defendant chooses to testify, the People may impeach his credibility by showing that he has previously been convicted of a felony. Even here, however, impeachment evidence of prior felony convictions must be limited to identification of the conviction, and `the courts will be zealous to insure that the prosecuting attorney is not permitted to delve into the details and circumstances of the prior crime [citation]. . . .' [Citation.]" (People v. Schader (1969) 71 Cal.2d 761, 773, italics added.)