Is my Time Machine backup encrypted or not?
Solution 1:
I deleted the old backup and created a new one, with encryption on:
Yes, the checkbox in the above screenshot is displayed as checked if the backup is encrypted.
Solution 2:
Make sure the driver is partitioned in GUID format. If you do elect to reformat it as GUID, this will erase your data. Also, network backups can only be encrypted before the first backup is carried out.
Hover over the greyed out checkbox and I think you should be able to see an explanation.