Dunno if this adds anything to the other excellent answers, but if you get an altimeter, as bookworm says, place it at ground level at your airfield and twiddle the knob till it shows the airfield elevation, the subscale reads the QNH.
Dig a hole from a/f level down to sea-level, and drop the altimeter down it.
Because of lapse rate differences etc., it is very unlikely that the alti will now read zero. Confused?
