Checking a level's accuracy
If you drop your level or doubt its accuracy, the best way to check it is to place the level on a relatively level surface. Shim up the low end until you get a level reading. Make sure to stand perpendicular to the vial when looking at it. Once you get a level reading, turn the level around 180 degrees. If the vial still shows level, you're getting accurate readings.
Check the vial that gives you a plumb reading by placing the level against a door frame. Tape paper shims to the jamb until it's plumb, then flip the level upside down and check for plumb again.
Some levels have vials that can be adjusted. But our advice is to buy a new level if yours is out of whack.