How to check encryption status (completeness) of an external disk?
If you type the following command in Terminal you will get all sorts of info on all the corestorage volumes currently attached to your Mac, including the conversion status you are looking for:

diskutil cs list


There are more answers here.

One of my favorites is Nick Bedford's answer towards the bottom of that page that gives you just the status of each volume without all the other info.

diskutil cs list | grep -e "Conversion" -e "Volume Name"