What is the length of Fiber optical cable. If it is not much or no joint is there in between than simple light transmitter and receiver module can help.otherwise go for OTDR (Optical Time Domain Reflectometer) is a test to be done end to end. it can be done by specialists and test report will be provided to user. this test help to locate the cable damage location if it is strectched in several Kms.i am not sure about minimum DB it can depend on length, number of joints, but we consider 7 DB was the last allowable limit for our system.