When you add a new citation, you will have a chance to select a Source. That is a drop-down list at the top of the screen and will show you the sources you have - yours and Ancestry's.
From that page, you can also add new sources or change existing sources.
If you want to just see the list, cancel out of that screen without adding a new citation.
Yes, it is safe to sync. I have a large tree and had more than 5,000 sources deleted in the recent clean-up fix.