How can you know about edge cases without knowing lower-level implementation details? How can you test behavior for a unit without understanding the code which

How can you know about edge cases without knowing lower-level implementation details? How can you test behavior for a unit without understanding the code which comprises the unit? One of the tenets of TDD is not testing obvious things, and there are bunches of obvious things which can permanently eliminate corners from an implementation. Overtesting is a real danger if one dogmatically forces their tests to specify the limits of the API.