@Gregg...If you're unsure of whether your published epubs have KFX enabled then you should just run your original, uploaded epub versions through Kindle Previewer 3(KP3). And if those epubs have "Enhanced Typsetting" enabled in KP3 then they will be converted and displayed as KFX on any Kindle ereader that can currently handle KFX. Here is a general list of Kindle ereaders that can handle KFX:
* Kindle E-readers 6th generation and newer
* Fire tablets 3rd generation and newer
* Kindle for iOS v4.9 or newer (iOS Versions: iOS 7, iOS 8 and iOS 9)
* Kindle for Android v4.18.0 or newer (Android Versions: Jelly Bean, KitKat, Lollipop, and Marshmallow)
Source:
KJ Enhanced Typsetting
If you can't run KP3 on your Linux computer then here's another idea. If you've already purchased and downloaded author copies of all your ebooks and downloaded them onto your Kindle reader(s) -- which must also have KFX capability(see list above) -- then just find the actual book file on your Kindle Tablet and check if it has a ".azw" or ".azw3" extension. If your ebook has the ".azw" extension then it has been converted to KFX. But if it has the ".azw3" extension then it will be formatted as KF8.