Which Microsoft word version a .docx file was saved?
Make a copy of the *.docx file and change the extension to *.zip.
Open the zip file.
Open the docPropsapp.xml file.
Look at the end for the AppVersion tag value:
12.0000 = Word 2007
14.0000 = Word 2010
15.0000 = Word 2013
16.0000 = Word 2016