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

"'"Both federal due process and state law require a trial judge to suspend trial proceedings and conduct a competency hearing whenever the court is presented with substantial evidence of incompetence, that is, evidence that raises a reasonable or bona fide doubt concerning the defendant's competence to stand trial."'" (People v. Sattiewhite (2014) 59 Cal.4th 446, 464.) "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.) "The failure to declare a doubt and conduct a hearing when there is substantial evidence of incompetence, however, requires reversal of the judgment of conviction." (Ibid.)