Less of an issue these days since a CC account will auto-convert them in the Cloud and the prompt usually gives you a clue, like CC2015, but not exact, like CC2015.2 etc.

But if you don’t have CC, have a need, or are just curious exactly what version was used, you can just open the ID file in a text-editor like TextWrangler choosing “Enable everything” and search for @1 to find first version it was saved in, and the final version will be at the end of that list of versions. Ugly code to wade thru but you’ll find the version number.

You can also use Word, choose “all files” and then unicode and and Western encoding (here in USA at least).