The level is a tool that has been around since the ancient Egyptians built the pyramids, but the Stanley level has only been around since the 1800s. The basic level is a symbol of masonry, used by builders the world over. In order to read measurements on any basic level, you will have to understand that the heart of the level is the vial. If you can read the vial, you can read the level accurately.
Reading the Vial on a Basic Level
Vials are "level" when the bubble is in the center, or in between the lines which indicate the center. That is the basic format, but there are two other types of vials: curved and pitch.
If using a curved vial within a wood level, the bubble will move slowly. That way you can work in terms of the bubble's location within the vial, and the bubble will rest at the top of the vial.
If using a pitch vial, which is found in aluminum levels, there will be pitch markings rather than the two lines which indicate that it is indeed "level". With a pitch vial, "level" is indicated when the bubble touches the zero line.
Checking the Accuracy of Your Level
Place your level on a flat horizontal surface that you know to be level.
Take note of the bubble's position within the vial and mark that position on your level surface area.
Turn your level to its side, or reverse it, and make sure to place it exactly where you marked on the surface.
If the bubble is in the same position as originally marked, then your level is accurate.