Five ways to know when you're done refactoring & testing

1. Test until bored. Ask "is there any chance this is wrong?" If so, write a test for it. If not, you're done.
   
2. If the test and the system-under-test would look the same, skip the test, because of DRY.
   
3. If the code reads like a spec, test by inspection: read it and confirm that it says the right stuff.
   
4. If a function has a side-effect-free query, followed by an action based on the results of the query, split them and test separately.
   
5. I'm not very good at figuring out how to design code well, but I can tell when code is hard to test. making code easy to test usually works out well for me. If I have to mock A to test B, that's harder than factoring out B and testing it directly. So, I refactor.

These are all different ways of saying pretty much the same thing.