In my job, I’m a very heavy user of Google News. Sometimes I need to know whether a particular news source is among those feeding Google News, so that if it isn’t, I can visit/subscribe to that source separately.
Bad news: Google treats this information as proprietary. Good news: somebody else has compiled a list by analyzing a zillion Google news search results.
Newsknife, a New Zealand-based Google news ranking service, maintains what seems to be the most complete and up-to-date list of Google News sources. It’s not perfect–there’s a chance they’ve missed some, and any sources Google has dropped (or which have closed up shop) may still be on the list.
Another alternative, if you just want to confirm whether a known source is feeding Google News, is to do an advanced search on Google News, and fill in only the name of the ‘News Source.’ If there are any search results, Google (obviously) is using that source. If there are no results, then Google is probably not.