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.