We calibrated our EFIS compass by taping a Silva compass to the wing & rotating the aircraft through the cardinal points. We checked the heading using a sighting compass from behind the aircraft.
You can check your calibration by taxiiing in a straight line and seeing if your GPS track & EFIS heading are the same (Check whether your GPS track is shown magnetic or true - ours shows a Mag tack - but if true you'd expect a difference of the magnetic variation).