How can I verify that my Photos and iPhoto library are hard linking to the same disk data?
Hard linked files share the same inode (which kind of identifies the file in the filesystem). So what you can do is to get the path of two photos which should be the same ("Reveal in Finder" in the old application, probably something similar in the new), open Terminal and run

ls -li /path/from/old/app.jpg /path/from/new/app.jpg


to see whether the number is really the same.