An inaccurate spirit level is the worst kind of adviser, able to ruin a building project before a single nail has been driven. To check that your level is on the level, place it on a tabletop and shim it with cardboard until the bubble rests at dead centre. Now turn the level over, and rest it on the shims. It should be perfectly level. Then flip the level end for end, and perform the same tests. To check for vertical accuracy, hold the level against a wall, and tape shims to the wall until the bubble reads plumb. Turn the level, then flip, as above.
Unless your level can be adjusted, dispose of it immediately if it is even a bit out of whack, or you’ll continue to use it for chores. (A 2′ level that’s out by 1/4″ translates into a 2″ error on a 16′ deck.) If you drop a level, or suspect it’s telling lies, do the level shuffle: check, turn, check, flip, check, turn, check.
If you must keep a bad spirit level around to use as a straight edge, at least poke out its eyes (smash the glass vials) so it can’t spread any more lies