We have a commercial metal detector along with several sensitivity calibration standards. We test paper and board products that are suspected of containing metal. We check the entire sample, then, if metal is detected, we halve the sample and test both halves separately. If metal is found in one half only, we then halve that sample again and repeat the process until we get to a postage stamp size piece containing the metal. Then we go to the microscope and visually find the metal. This test is very useful when filled food cartons are being rejected since we can prove the presence of metal if it is in the paperboard.
We can also manipulate the sensitivity of the metal detector to determine the level at which known metal will set off the alarm.