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

"A trial court's decision whether or not to hold a competence hearing is entitled to deference, because the court has the opportunity to observe the defendant during trial." (People v. Rogers (2006) 39 Cal.4th 826, 847.) We review a trial court's decision whether to grant a competency hearing for an abuse of discretion. (People v. Ramos (2004) 34 Cal.4th 494, 507.)