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. [Citations.] ... Evidence of incompetence may emanate from several sources, including the defendant's demeanor, irrational behavior, and prior mental evaluations. [Citations.] ( People v. Rogers, supra, 39 Cal.4th at p. 847 [48 Cal.Rptr.3d 1, 141 P.3d 135].) But to be entitled to a competency hearing, a defendant must exhibit more than bizarre ... behavior, strange words, or a preexisting psychiatric condition that has little bearing on the question of whether the defendant can assist his defense counsel. [Citations.] ( People v. Ramos, supra, 34 Cal.4th at p. 508 [21 Cal.Rptr.3d 575, 101 P.3d 478].) ( People v. Lewis, supra, 43 Cal.4th at p. 524, 75 Cal.Rptr.3d 588, 181 P.3d 947.)