Final answer:
The M'Naghten Rule is the standard currently used to determine legal insanity, with competency to stand trial being assessed by cases such as Dusky v. United States.
Step-by-step explanation:
The rule currently used to determine legal insanity is the M'Naghten Rule. Legal cases such as Dusky v. United States have established that a defendant has the right to a competency evaluation before proceeding to trial. Dusky established a standard for determining when a defendant is competent to stand trial, which incorporates understanding the proceedings and being able to assist in one's own defense. Cases like Ford v. Wainwright and Godinez v. Moran further assert the necessity of competency evaluations before execution, pleading guilty, or waiving the right to legal counsel.