Levels are always assumed to be level. But as a big tool, they can get banged up and become out of level as a result. If you want to know if your level is actually level, it's really easy to check.

Find something flat and put your level on it. If needed, shim the thing your level is on until the level indicates that it is in fact level. Then swing your level around (switch the ends). If you level is truly level, it should still read level. If the reading is different after you switch the ends around, your level is no longer readin level.