As a fan of unit tests for bug checking and development (where applicable) I always have a doubt that even though my tests pass they might not be written correctly. This is especially true where you write the tests after development rather then following TDD.
Regardless you can still stress your code by doing some mutation testing. It’s essentially a way of testing your tests. Quite a few mutation testing frameworks are out there such as Heckle, Insure++, Nester etc.. but I was looking for one for my Python and PHP code that goes into making searchco.de
Usage is pretty simple,
python mutator.py DIRECTORY EXTENTION
Where directory is the directory you want to recursively target and extension is the file extension you want to target. You can get the code here at github.