Test for Accuracy
The field-test for level accuracy is so quick that I perform it almost every time I use the tool.
Step 1. Place a level on a flat surface and read the bubble position in relation to the lines on the vial.
Step 2. Keep the level horizontal, pivot it 180 degrees, and place it back in the same spot. If the bubble returns to the same position in the vial lines, then it's accurate. Repeat the test for each vial in the level. (For the vials that measure plumb, like the one I'm looking at in the picture, flip the level onto its opposite edge.) If one vial is off but the rest are consistent, you can mark the bad one so you'll know to ignore it.