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

A defendant is presumed to be mentally competent to stand trial. ( 1369, subd. (f); People v. Young (2005) 34 Cal.4th 1149, 1216.) In California, "[a] defendant is mentally incompetent . . . if, as a result of mental disorder or developmental disability, the defendant 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).)