Place the level on a flat surface.
Make one mark at the end of the level.
Make another mark along the side of the level, under the vial in the center.
Take a reading of the bubble's position.
Rotate the level 180° end-to-end and align the level with your marks.
Take another reading. If the level is accurate, the bubble will be in the same position for both readings.
To test the vertical vial, follow the same procedure against a flat vertical surface.