For lack of a better term Bismuth is diamagnetic Unlike steel which is sucked into a strong magnet Bismuth behaves in the other direction. If you have a flat disc of Bismuth and place a strong magnet on it the magnet will seem to levitate upwards. You can check to see if shot is Bismuth by seeing if a magnetic field is repelled by it. Steel is attracted, Bismuth is repelled and lead is not effected.
So all a game warden needs is a gizmo which emits a magnetic field and see if it gets distorted to a plus direction or a minus direction. Plus is steel being attracted toward the field, minus is Bismuth being repelled in effect. The gizmo just shows lights, green for steel, red for Bismuth. If neither it could be lead or some other non tox shot. Different gizmos use different combos but they all work along these simple lines. You can use a compass to see the same thing. Steel shot will attract the needle Bismuth will make it shift 90 degrees. Kind of neat.
