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

The statute provides that, on the issue of mental competence to stand trial, the court "shall appoint a psychiatrist or licensed psychologist, and any other expert the court may deem appropriate, to examine the defendant," and, "[i]f it is suspected the defendant is developmentally disabled," the court shall also appoint the director of the regional center for the developmentally disabled to examine the accused and provide the court with an expert opinion as to whether the accused is competent to stand trial. (Pen. Code, 1369, subd. (a).) A purpose of this requirement is to ensure that a developmentally disabled person is evaluated by experts experienced in that field and to thus enable the trier of fact to make an informed determination of the person's competence to stand trial. (People v. Leonard (2007) 40 Cal.4th 1370, 1391.)