To check the accuracy of your GPS, contact your local government agencies that deal with maps, mapping or land titles,
or a survey company in your area and ask for the coordinates of a local survey pin.  You will need to know whether
the coordinate references you are given are UTM or Longitude & Latitude.

For example:

UTM coordinates should be similar to this format:
                N 5587076.776    E 525532.488

LONGITUDE & LATITUDE in Degrees, Minutes, Seconds should be similar to this format:
               N50 26 14
(Exact N50 26 13.64161)    W104 38 26 (Exact W104 38 25.71283)

One second of error equals approx. 102 feet in distance. 

Elevation is generally shown as being above sea level in whatever units (feet or meters) of measure you have
chosen for your GPS.