What is the test for determining whether a defendant is mentally competent at a preliminary hearing?

Due process requires that a defendant be competent at all times during criminal proceedings. (Pate v. Robinson (1966) 383 U.S. 375 (Pate).) A defendant is 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).) If a defendant was mentally incompetent at a preliminary hearing, any witness testimony from that hearing is not admissible in a later legal proceeding because the defendant is not considered to

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