A simple way to test for mag compass error and DG precession is to use your
GPS heading indication while tracking on he ground (i.e. taxiing as opposed
to flying).

Find an airport with some clear apron surface that you an use. Using the
GPS, taxi straight north (hdg 360). Use enough power to keep he vacuum
indication "in the green". Set DG to 360, and note mag compass indication.
Make successive 30 degree turns, according to the GPS. After each, note the
DG and mag compass indications. If the DG fails to track new headings
(within tolerance) you have excessive precession. Mag compass errors can be
used to fill in a correction card, or if they are excessive they indicate a
need to make adjustments.

Doing this on the ground eliminates the issue of wind (unless it is really
howling to the point that the airplane does not track straight while
taxiing).

--
-Elliott Drucker