OG GarlicSalt

We swing our compass every 3 years another method you can use is the "Moses compass" not sure the proper name. 

 

This tool goes on a pole and you align the front and back of the aircraft with the sights, like a gun sight. Turn t the aircraft until the mag compass in the aircraft reads dead north for example walk out in front 30 or so feet then check it against the tool it's already 180 flipped so even though it's faced the opposite way it will read the same as the aircraft. If the aircraft compass doesn't read the same as the tool that's your deviation.