What is the test for "express malice" in a murder case?

Murder is the unlawful killing of a human being with malice aforethought. ( 187, subd. (a).) Malice is express when "there is manifested a deliberate intention unlawfully to take away the life of a fellow creature." ( 188.) "[U]nlawful 'intent to kill' is the functional equivalent of express malice. [Citation.]" (People v. Swain (1996) 12 Cal.4th 593, 601.) "[O]nce the trier of fact finds a deliberate intention unlawfully to kill, no other mental state need be shown to establish malice aforethought." (People v. Saille (1991) 54 Cal.3d 1103, 1113.)