When running phpunit you have the option to show how much of your code is covered by tests and which parts aren't.
To do this just run phpunit with the following flag:
$ phpunit --coverage-html path/to/folder
This command will create a documentation of your code coverage and save it into the folder you specify. An existing folder will be overwritten on second execution.
Even if you already got tests in your code, you going to be surprised how many parts aren't covered yet.