Dev: How to find out which URL the response is from?

I am trying to follow http://astroquery.readthedocs.io/en/latest/index.html#developer-documentation in order to transplant Cone Search from Astropy to here. In Cone Search, I need to know which service URL that is attached to a response. However, in your API, the arg parsing and the response handling are separated into two methods. Your URL attribute is a list of mirrors (or in my case, a list of different services to try in order), then you pass this whole thing into the `GET` stuff, and then suddenly I have response from one of them, but which one? How do I find out?

Read the linked Astroquery developer documentation and the Cone Search work being transplanted from Astropy. Trace how the list of mirrors or services is passed into GET and how the selected response is exposed. Done means the project has a clear, agreed way to identify the service URL associated with a response.

Written by the indexing model from the issue text.