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

Due process prohibits trying or convicting a defendant who is mentally incompetent. (People v. Rogers (2006) 39 Cal.4th 826, 846.) "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).) "It shall be presumed that the defendant is mentally competent unless it is proved by a preponderance of the evidence that the defendant is mentally incompetent." ( 1369, subd. (f).)

"In reviewing a jury verdict that a defendant is mentally competent to stand trial, an appellate court must view the record in the light most favorable to the verdict and uphold the verdict if it is supported by substantial evidence. [Citation.] Evidence is substantial if it is reasonable, credible, and of solid value. [Citations.]" (People v. Marshall (1997) 15 Cal.4th 1, 31.)