Let's say for a steel bar, you have the frequency information on each spatial location along the steel bar, could you make a decision that a defect/crack is exist on the specific location without having to inspect the steel bar first?
The answer is yes. Having information on natural frequencies and mode shapes, such as you could gather from a series of accelerometers, could allow you to detect defects. However the resolution might not be all that great. I.e. you may be able to detect a large defect but miss a small defect. Depends on what size defect is considered acceptable. If you need the best resolution, there are probably better methods such as ultrasonic. Also mounting a lot accelerometers will be time consuming. Ultrasonic would be much quicker.