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