My favorite is the "Box" test to see how accurate it returns to zero. You know, so many clicks down, shoot, same number clicks left, shoot, same number clicks up, shoot, same number clicks right, shoot, same number clicks down, shoot: measure the track of the shots.
Do a torture test of the ON/OFF switch while spritzing it with water: click it 50 times ON/OFF while hitting it with a light-but-steady-mist.
Check to see how well its interior is nitrogen purged by taking it from dry cold to moist heat and vice versa. I see an oven on loooow heat, a bathroom with the shower running, a freezer, and a cooler with icewater playing major roles in this.
Look at its dot in all the lighting conditions you can possibly come up with: total dark, bright light, parking lot with sodium vapor lights, moving from a dark corridor to a room with bright sunlight ("I cleared buildings with these"), etc.
(Do this last) Drop it from 4 feet onto its side - twice - then see what happens.
Hope some of these seem helpful!