Place the level on the floor or a table. Mark its exact location at each end with a pencil (mark the line on the end and a short distance along each side), and note the bubble reading on the vials.
Turn the level around, end for end, aligning it with the pencil marks. Check the reading. Repeat, by turning the level upside-down. All readings should match. If they don’t, adjust the vials using the screws at each vial and repeat the test until all readings match. If your level does not have screws to adjust the vials, you’ll just have to buy a new level.
Before you buy, try this test.