What is the test for determining whether a defendant is competent to stand trial?

If a defendant offers substantial evidence of incompetence, refusal to hold a hearing is a denial of due process and is per se reversible error. (People v. Pennington (1967) 66 Cal.2d 508, 511. However, "'The court's decision whether to grant a competency hearing is reviewed under an abuse of discretion standard. [Citations.]'" (People v. Oglesby (2008) 158 Cal.App.4th 818, 827, quoting People v. Ramos (2004) 34 Cal.4th 494, 507.)