On the back of the compass card is the signature of the person who adjusted the compass and the date. It's nice to know that it has been done. A number of years ago an aeroplane at the school where I worked suffered a catastrophic engine failure and the instructor just managed to get it back on the runway with oil spewing everywhere. Our maintainance engineer (who did not work on the field) turned up in his van a few hours later with a box of tools and a new engine in the back. This he proceeded to fit and once the work was completed I was asked to do an air test before the compass was re-swung. The engine behaved perfectly, but as I lined up I noted a 30 degree error in the compass reading. After the test, half an hour on the airfield's compass circle and the compass was once again reading in the region of 1 degree from the magnetic heading. This was in the days before reliable GPS units and I certainly would not have liked to take an aircraft on a cross country with such an error.
P.P.