If you get a chance to work with one of the "transparent" fibers (no signifigent jacket) you normally don't see any "leaks" along the way -- the fiber will appear "dark". However, if you create a too sharp bend, you will be able to see leaks at that point. You can also see this if you strip away some jacket on a regular fiber, then kink the exposed fiber. You'll be able to see leakage at the kink.