What is the test for due diligence in a criminal case?

"[T]he term `due diligence' is `incapable of a mechanical definition,' but it `connotes persevering application, untiring efforts in good earnest, efforts of a substantial character.' [Citations.] Relevant considerations include `"whether the search was timely begun"' [citation], the importance of the witness's testimony [citation], and whether leads were competently explored [citation]." (People v. Cromer (2001) 24 Cal.4th at 889, 904.)

"[A]ppellate courts should independently review a trial court's determination that the prosecution's failed efforts to locate an absent witness are sufficient to justify an exception to the defendant's constitutionally guaranteed right of confrontation at trial." (People v. Cromer, supra, 24 Cal.4th at p. 901.)