Xcode files ARC setting

To see which files override the ARC setting:

Select your project in the navigator. This will open the project editor.
In the project editor, select your target.
In the project editor, click Build Phases (not Build Settings).
Expand the Compile Sources build phase.
Here, you can see the compiler flags for each file. Files with that set whether or not to use ARC will include either -fno-objc-arc or -fobjc-arc, for specifying MRR or ARC respectively.