You have been working on a feature branch, and you want to find the differences (across the two branches).
Make sure that you the the latest version of the feature branch checked out.
Go to the “Git branches” menu (which is the bottom right of the screen, or via VCS -> Git -> Branches.
Select a branch, and choose compare. If you select the “diff” option - you will get a list of files which are different on the two branches.