One good low-light focus trick I've heard, replicating the old Kalart Focuspot trick, is to aim a laser pointer through the eyepiece of a rangefinder on a rangefinder camera (obviously not aimed at someone's face). This will project two spots, and when the spots are in line on the subject, the subject is in focus. I've tried it with a small LED flashlight, and it works in principle, but one spot is fuzzy and one is sharp, making it pretty imprecise. The laser pointer is supposed to solve the fuzziness problem.