The official name for a level is a "spirit level." "Spirit" refers to the liquid (usually ethanol) in the glass tube that gives you the degree reading of how in or out of level an object is. An engineer's spirit level can be calibrated. Unlike cheaper levels, an engineer's spirit level allows you to check and correct imbalances in the level each time you use it. Cheaper levels do not allow you to calibrate, and you may not know, until it is too late, that their "spirit" is off. Calibrating a level is relatively simple, but it may take a few tries to get used to what you are looking for.
Place your engineer's spirit level down on the flat surface as if you were going to read its level. A small table works very well as it will be easy to adjust the height of the table by lifting it and shimming under the legs if necessary.
Let the level sit for 15 to 20 seconds. This will allow any vibration from the movement of your placing it on the table to dissipate and the reading of the table's level to be true.
Note where the centre of the bubble is on the scale printed on the outside of the glass vial in the level.
Turn the spirit level over so what had been the top edge is now on the bottom. Wait another 15 to 20 seconds for the level to settle and note the position of the bubble. If the bubble is centred in both readings, then your table and the level are calibrated. If the bubble is in a different position in either reading, go on to the next Step.
Note which end (right or left) of the glass tube the bubble is closer to. Flip the level, putting what had been the top edge on the bottom, let it settle and see to which end the bubble is now closer to--if it has drifted towards the same end (i.e., to the right in both readings), then your surface is out of level. Shim the legs of your table until the bubble is centred in the tube and then flip the level and take another reading (beginning the steps again according to whether the readings match or not). If the bubble moves toward the opposite end in the second reading, then your level needs to be calibrated.
Locate the calibration screw on your engineer's spirit level. The screw will be in one of the edge faces, near or directly over the glass tube. Set your level on the table with the edge with the screw facing up. Use a small Phillips screwdriver to turn the screw, very slightly, to calibrate the spirit level. As you turn the screw, you will see that the bubble will begin to move one way or the other. Keep turning the screw until the bubble is centred.
Always use the same flat surface to calibrate your level on. This way you will already know if it is in or out of level and how it needs to be adjusted, and will not have to figure it out each time you calibrate.
Use caution when turning the calibration screw clockwise. This tightens the screw into the spirit level casing, and can crack the casing and ruin the level if turned too much.