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

"A person cannot be tried or adjudged to punishment while mentally incompetent. ( 1367, subd. (a).) A defendant is mentally incompetent if, as a result of a mental disorder or developmental disability, he or she is unable to understand the nature of the criminal proceedings or to assist counsel in the conduct of a defense in a rational manner. (Ibid.) When the accused presents substantial evidence of incompetence, due process requires that the trial court conduct a full competency hearing. [Citations.] 'Evidence is "substantial" if it raises a reasonable doubt about the defendant's competence to stand trial.' [Citation.]" (People v. Lawley (2002) 27 Cal.4th 102, 131.) "On appeal a finding of competency to stand trial 'cannot be disturbed if there is any substantial and credible evidence in the record to support the finding.' [Citation.]" (People v. Hightower (1996) 41 Cal.App.4th 1108, 1111.)