See Also, Vial Types
The great thing about bubble vials is that they are for the most part easily checked for errors. Rotating your level like a top to check the opposite side or turning it over end for end and then rotating it will give you a 4-way check for accuracy.
Most importantly for this check you must be sure the level is reading from exactly the same points each time.

Remember, If your levels metal (or wood) frame is twisted, crowned, or bent, it cannot accurately give consistent readings all 4 ways. So eyeball your level from the end like you would a stud to see its straightness before you begin.
Method 1. Using the bubble itself as a witness (methods a->c)
a. Select two outstanding points, roughly the length of your level that its ends can be rested against and the vials easily read (a door jamb or its hinge pins protruding beyond the door, or a nail or screw in the bottom plate of a fire blocked stud bay will work)
Place the level against the points and read it. Note the bubbles location, rotate the level edge for edge (not end for end) and read it again. If the bubble is not in the same location in relation to the reference lines, the error is _ the distance they differ.

b. You can also draw a line (one end at a fixed point, the other end receiving the line) that represents a centered bubble from your levels 1st edge. Rotate the level and draw a second line. Any error is the distance between the two lines.
c. Also, using one fixed point as baseline, move your level sideways until the bubble just touches its reference line , draw a pencil line along your levels working edge. Spin the level and do the same using the bubbles other reference line. The distance between the two lines is the bubbles error. A centerline between the two lines should be perfectly plumb.

Method 2. Using a plumb Line
Using a plumb line (fine wire or fishing line) is another way to check your levels vials.

Since it’s hard to get your level against the line, hang the plumb line about 1-1/2” away from a wall, use a light placed perpendicular to the wall to project the line (use a fine wire, string, or fishing line to give the clearest line). Use a sharp pencil to mark points along the projected line.

Remove the plumb line. Hold your level’s edge(s) to the points and check the vial for centeredness. This only works for plumb vials (not horizontals).

See Also, Vial Types