We used to lay the bag over the mixer (for manual handling ease) and slit the bag as you describe, however we had a suitable (4 micron) sieve housed in the neck of the mixer top. Then at a set frequency (After every addition/bag/batch/1000kg depending on your process) the sieve was inspected for FB and damage. This inspection was recorded as part of our HACCP it was a control point. The size of sieve depends on what materials you are adding to the mixer and should be optimized for grain size and process speed. I.e for brown sugar or chocolate chunk products we had a larger sieve than for flour. We also had a shaker on the neck to aid passage of the material.
Corrective action; The sieve captures any paper bag but you should also look for other items which should not be reasonable expected to be there. This will indicate problems are occurring. We once found wire in the sieve which had come from a bag of onion flake. This lead to a supplier complaint as the wire was originally on the bag of onions they had dried, flaked and supplier to us. It will also capture and FB form your equipment. Depending on what you find in the sieve you make a judgment based on risk assessment and take the relevant actions.