ok. so you have a spirit level which you inherited off grandad or bought as an ebay bargain. you want to see if its level reading is correct?
this only works for the spirit bubble type levels, the ones with a visible air bubble in a tube or a dome if its a bulls eye type level.
easy. find a flat surface, not necessarily perfectly level but flat.
put your spirit level on the surface and make a note of where the bubble lies.
now turn your level around, through 180 degrees , the bubble should be in the same position if your level is accurate.
to double check find a surface that should already be levelled eg a kitchen worktop. check your level on it firstly pointing left to right then turn around to point right to left. if your level is accurate the bubble should show in the centre exactly both ways.
most levels have screws which can be loosened to re level the bubble vials for accuracy.