I presume you have the spirit level position vertically and are using one of the vials that are position at right angles to the surface.

If you hold one end against the wall and move the other end of the level away from the wall to the position where the bubble is perfectly central, then look at how far the level is from the wall. If it's a tiny distance then fair enough, if it's a finger's width of something you know you've got a problem!

Imagine the wedge-shaped gap between level and wall and how its bigger end would grow wider if you were to have a level that was six feet long.