What is the test for determining whether a defendant is mentally competent to assist his defense?

. . . behavior, strange words, or a preexisting psychiatric condition that has little bearing on the question of whether the defendant can assist his defense counsel. [Citations.]" [Citation.]' [Citation.]" (People v. Sattiewhite (2014) 59 Cal.4th 446, 464-465.) Accordingly, "defense counsel's expressed belief that [the] defendant might be mentally incompetent does not automatically trigger" a requirement that the trial court order a competency hearing. (Id. at p. 465.) Instead, "defense counsel must present expert opinion from a qualified and informed mental health expert, stating under oath and with particularity that the defendant is incompetent, or counsel must make some other substantial showing of incompetence that supplements and supports counsel's own opinion. Only then does the trial court have a nondiscretionary obligation to suspend proceedings and hold a competency trial. [Citation.] Otherwise, we give great deference to the trial court's decision not to hold a competency trial." (Ibid.)