Hi , just my tuppenny's worth.
FOD control is much like Safety, it is the proactive evidence of control that is more important than evidence that FOD has not caused damage to aircraft or equipment. I say like safety because you're not sure whether it was your effective safety management system that prevents accidents or just luck.
You therefore need to demonstrate that you have an effective FOD Control programme by demonstrating effective tool control and accountability, regular clean-ups for work benches, hangars and tarmac, regular FOD training and awareness (part of recurrency training) and ultimately no FOD related damage events.
As James Reason suggests in his numerous books, it is better to monitor the number of safety meetings, safety training events, near miss reports and demonstration of a just culture that ultimately measures how good a safety system is. A good FOD Control system should follow the same process.
Also, FOD is not addressed very well in regulatory requirements, particularly FAA and EASA 145 regulations (just try and find FOD in the regs or the AC's). AS9100 and AS9110 include it as a welcome addition.