How can I tell which photos library my Photos application is using and where it lives?
___________

You can find out which photos library is in use as well as if it's the system photos library (the one syncing to the cloud) by opening Photos, going to Photos preferences and selecting the General tab.



There you will see the file path to the library Photos is using. Just below that, if the "use as system library" button is greyed out, that means it is the one being used to sync to the cloud. If it is black and clickable, that means it is not currently the system library.