If it reads differently end for end, on a true level surface, it is out of adjustment. If the bubble "centers" on a true level surface it is properly adjusted.
If you don't know if you have a true level surface and you don't know if the level is properly adjusted then you don't have any useful info no mater what the bubble does and you need to adjust it. This can be done even without a true level surface by observing where the bubble is for each of the two positions and adjusting the level so it is "off" the same amount for each.