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

Penal Code "[s]ection 1367, subdivision (a) states that a 'defendant is mentally incompetent . . . if, as a result of mental disorder or developmental disability, [he] is unable to understand the nature of the criminal proceedings or to assist counsel in the conduct of a defense in a rational manner.' ' "When the accused presents substantial evidence of incompetence, due process requires that the trial court conduct a full competency hearing. [Citation.] Evidence is 'substantial' if it raises a reasonable doubt about the defendant's competence to stand trial." ' " (People v. Weaver (2001) 26 Cal.4th 876, 953.)