If either vial is not true you put it down and try another.
What if both vials are out?
Preston only store
To test horizontal you put it on a shelf and look at the reading. You then turn it around 180 degrees and read it again. If the readings are the same, both ways round, then the level is true. To test the vertical is obviously the same, but with the level placed against an upright. Last time I was shopping for a level, some years ago, it was shocking how many cheap ones were really out of true.
How the hell are you supposed to makesure it reads true?