Set the level on any close to level surface and mark the location of the level.
Rotate the level 90? in the same location and the bubble should be in the exact location it was in the first test, if it is the level is ok.
A easy test the level hold the level on the wall with the bubble between the lines then draw a line on the wall.
Rotate the level 90? and check the line, if it is not level then draw a new line with one end on the first line.
Level is a line in the center of the distance between the two lines drawn.
A good idea if you are married put a piece of paper or removable tape on the wall first.
Originally Posted by larosta
You may need to check the calibration of your bubble level. A web search for ?how to calibrate a spirit level? should return several written and video procedures. If your level is out of calibration and Unless your level is a expensive engineers machinists level you will probably just want to replace it.