The Ninth Circuit has elaborated on the standard for demonstrating mental incompetency.  To demonstrate mental incompetency, a person must show some inability to comprehend or to assist and participate in the proceedings, some inability to consult with or assist their counsel or their representative if pro se, and lack of a reasonable opportunity to present evidence and examine witnesses, including cross-examination of opposing witnesses. The mere inability to recall some events, a common weakness, and other similar mental lapses, are not sufficient to show mental incompetency, if they do not show any inability to understand the nature and object of the proceeding.

The full text of Mora Salgado v. Sessions can be found here: