How do you tell if a FM file is a 2017 file vs a 2015 or earlier file?
I don't think there's much that identifies it with a particular version other than trying to open it - it should pop a message about the version if it's older or newer.
Ensure your preferences aren't masking the Opening an old release document alert,
then open the file in Fm 2017. If no alert, then it's an Fm2017 doc. If an alert, then it's a previous release and the original author may not be able to access if you save changes.
From a quick check on one file this morning … opening an .fm file in a text editor shows the FM version as the first line:
Nice! I just did the same with a 2017 .fm file and found <MakerFile 14.0>