Which config is which?

You can use

AppDomain.CurrentDomain.SetupInformation.ConfigurationFile

to determine which app.config file is being used by ConfigurationManager.

Note that if you are writing a service then the service uses its own app.config file and not the one of the program calling it. This can lead to confusion when developing services in Visual Studio using a test driven development methodology as the app.config in the test project is not the one used by the service being tested.