What is the simplest way to list which attached disks are encrypted (and which not) on AWS?
To get the number of non encrypted volumes you can run this command:

aws ec2 describe-volumes --region &lt;your_region&gt; --filter "Name=encrypted,Values=false" --query "length(Volumes[])"


length will return the length of the array Volumes flattened by the selection operator [] (more details on JMESPath documentation).
As we filter the slection for non encrypted volumes (--filter "Name=encrypted,Values=false") this should allow to demonstrate to the auditor the number is 0 not encrypted volumes.

Same filter can be applied in the console, in the ec2 page, under 'Elastic Block Store' => 'Volumes', type Encrypted : Not Encrypted to filter the view to non encrypted volumes only. you may add Attachment Status : Attached to list only attached volumes.