The way I've checked for interference, deflection, is to place my compass on a flat surface and orient it so the needle points toward a cardinal direction. Once set up I then move the device or gear I'm checking for influence on the compass nearby and look closely to see if the needle has shifted.

Using this technique a friend found that his belt buckle has highly magnetized and could swing the needle 30 degrees. Which somewhat restored his confidence as a navigator after it was shaken on a trip when nothing showed up where he thought it was supposed to be. A magnetized belt buckle, and differences in how he held the compass, would explain the variable readings.

I read a case where a man's surgical implant, a plate a screws in his arm, would throw off a compass. The good news is that for most people in most places small deflections have little real effect. If you were blind shooting for a tiny un-land-marked water hole across eighty miles of salt flats a small deflection might get you killed. Fortunately most of us are in familiar territory with enough terrain clues to keep us going in the right direction even if the compass goes wonky.

Still, if you have a compass it would be best to have it accurate and to limit interference with its accuracy.