For one night, the big question in "Hamlet" won't be "To be, or not to be" but "Is he or isn't he?"—as in "Is the prince of Denmark mentally competent to stand trial for the murder of Polonius?" The Shakespeare Center of Los Angeles will present a Jan. 31 mock trial to determine the answer in a program created and presided over by U.S. Supreme Court Justice Anthony M. Kennedy. The legal-literary proceedings, which will be held at USC's Bovard Auditorium, will be run "as if it were a bona fide trial," says Ben Donenberg, the Shakespeare Center's founder and artistic director.