find all core patches I work on site that is drupal 7.20, I am sure previous team that worked on this project patch core in some places,I want to find all of patched they did, is there any way to find all patched they did?
one of my idea is find a clean drupal 7.20 core and compare it but I am not sure this is good solution
You can use the Hacked! module

This module scans the currently installed Drupal, contributed modules and themes, re-downloads them and determines if they have been changed. Changes are marked clearly and if the diff module is installed then Hacked! will allow you to see the exact lines that have changed.
Hacked! also provides drush integration so that you can see what files have changed from the command line.