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

U.S. at 180 (explaining that there are "no fixed or immutable signs which invariably indicate the need for further inquiry to determine [the defendant's] fitness to proceed"). The right not to be prosecuted while incompetent "spans the duration of a criminal proceeding." United States v. Arenburg, 605 F.3d 164, 168 (2d Cir. 2010). A district court must therefore "always be alert to circumstances suggesting a change that would render the accused unable to meet the standards of competence to stand trial,'" id. at 168-69 (quoting Drope, 420 U.S. at 181) (emphasis in original), an obligation that "takes on increased significance where, as here, a criminal defendant elects to proceed pro se," id. at 169. We review a district court's decision not to order a competency hearing for abuse of discretion. Id.