In my experience, this test or a similar test using two polarized lenses by themselves with a light source such as the sun is 100% accurate, there is nothing "magic" about the computer screen, it simply supplies a polarized lens for the back side of the glass, and a light source. The glasses are the second polarized lens for the front side of the glass to be tested. Generally what you're looking for is a pattern in the glass, stripes, blotches, a checker board pattern... almost anything that shows up as light and dark areas. Untempered glass won't show any of those features when subjected to this test.
I always like to tell people to try this test on a side, or rear window of a car since vehicle safety requirements mandate those windows in modern vehicles be tempered. It's a way for most people to have access to a know tempered glass panel to see the effect. Laminated safety glass front windshields are never tempered and therefore won't exhibit the pattern(s) tempered glass does.
IMO, it's best to do your own test before ever drilling.
