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

A defendant is mentally incompetent to stand trial if 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." ( 1367, subd. (a).) "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. [Citations.] . . . Evidence of incompetence may emanate from several sources, including the defendant's demeanor, irrational behavior, and

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prior mental evaluations. [Citations.]" (People v. Rogers (2006) 39 Cal.4th 826, 847.)

Defense counsel's belief that his client might be mentally incompetent does not automatically trigger a competency hearing. (People v. Sattiewhite (2014) 59 Cal.4th 446, 465.) Instead, "defense counsel must present expert opinion from a qualified and informed mental health expert, stating under oath and with particularity that the defendant is incompetent, or counsel must make some other substantial showing of incompetence that supplements and supports counsel's own opinion. Only then does the trial court have a nondiscretionary obligation to suspend proceedings and hold a competency trial. [Citation.]" (Ibid.)