I use a couple polarizing camera filters, any photography store should have them. Sunglasses can work too but the tint makes it difficult sometimes. You'll need two lenses, or filters, one on each side of the glass. I get the best results outside in the sunlight with a clear sky as the background. Rotate the filters until most of the light is blocked, then move the pair of filters across the glass pane you want to check . If you see light and dark patterns, or splotches in the glass while looking through the filters it's tempered and cannot be drilled. You can try this on the side window in a car since they are all tempered to see the effect (only the side or rear windows, NOT the front).
HTH,
Tim