How to tell what apps are not being used?
In iOS 11, you can set up your ipad to move unused apps. Go to Settings and then tap General. In General there is a page for iPhone Storage. Here there is an option to remove unused apps. If you enable this then unused apps will be removed.

Someone reported on this site that they had a problem with there iPad not accurately determining the last used date. That person was getting this "app never used" setting for all apps. I assume you want to avoid that.

If you scroll down on same page where you enable the removal of unused apps, there is a list of all apps on your device. Under the name of each app is the last used date. Make sure that that field does not have "Never Used" for apps you know you have used.