How to check whether a person has shared a file from Intranet to Internet?

Your question has the word 'file sharing'. It can be checked easily after knowing how one is getting access to the particular file.

If you are getting access to the particular shared file without internet connection in your PC, it appears that the file is shared via intranet or local LAN network for which internet is not required.

If you are getting access to the particular file on internet, i.e, through mail or through online storage like a cloud account , it appears that the particular file has been shared via internet. That's All.