How can I tell which data sources are being used in SSRS?
Okay, I got it.  Found this link which helped: http://social.msdn.microsoft.com/Forums/en-US/sqlreportingservices/thread/581c7068-0abe-49f9-a1a5-93e94f472641/

I distilled that information to this query:

select
  Catalog.name,
  cat1.Name datasource
from
  Catalog
  join DataSource
    on Catalog.ItemID = DataSource.ItemID
  join Catalog cat1
    on DataSource.Link = cat1.ItemID
where
  Catalog.Type = 2


Hope this helps someone else in the future!