In the 1920s, master carpenter Henry Ziemann invented the spirit level that we use today. The level features a glass tube filled with a liquid spirit except for a bubble in the tube. The low viscosity of ethanol allows the quick movement of the bubble, and the glass tube has two lines the width of the bubble, drawn at the center of the tube. This will indicate if the level being measured is level or plumb.
Place the flat edge of the level on the surface you wish to ascertain the horizontal. Ensure the glass tube with the bubble is laying parallel to the surface.
Remove your grip from the level and watch where the bubble comes to a stop. The position of the bubble will indicate if the surface is sloping in one way or the other, or if it is level.
If the bubble stops above the two lines, then the surface is sloping down one way and should be raised at one end. The surface should be tilted one way or the other until the bubble comes to a rest exactly between the two lines on the glass tube. At this point the surface is plumb or level.
Indicate whether an upright is vertical in a similar way. The spirit level will have another tube placed in a circular hole on the same tool. The glass tube will run perpendicular to the previous glass tube.
Place the edge of the level against the upright and adjust the upright until the second bubble is exactly in the centre of the two lines on that glass tube. At this point the upright will be vertical.