What is the test for finding a defendant mentally unfit to stand trial?

Defendant argues that the record contains substantial evidence that he was mentally incompetent. "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. Rogers (2006) 39 Cal.4th 826, 847 (Rogers).)