Start point is config.xml file, for example /app/code/local/Companyname/Extensionname/etc/config.xml file.
Then next step will be analyzing files. There will be 3 aspect of checking it.
REWRITES Check model/class/controller.
Rewrites => you should at least take a look into every defined model/class/controller rewrite within your config.xml file. however its important to understand what the class/method you are rewriting does and try to call its logic (if possible) via parent::nameMethod().