A lot of it boils down to another entry on that list > Use your brain IE, here is your confidence in our code (0...100%) and here are tools to increase that c

A lot of it boils down to another entry on that list > Use your brain IE, here is your confidence in our code (0...100%) and here are tools to increase that confidence (unit testing, integration testing, monitoring, etc.). At that point it is straightforward to say, "does this unit test increase my confidence?" and then go onwards. The thing is you need to learn to accurately gauge your confidence which can be difficult at times, especially when you are dealing with unknown unknowns, but if you iterate on it you get to a place where problems pop up in the areas you are least comfortable with and that is a good signal you're at least approximating it correctly.