The management GUI provides a way to determine if each of the arrays in a storage pool are encrypted.
Any array that is made after encryption has been enabled is encrypted by default unless you select to not encrypt the array at the time it is created. However, the arrays that existed before encryption was enabled are not be encrypted. There is no way to change an array that is not encrypted into one that is encrypted.
If you require that all of the arrays in the storage pool are encrypted, then you can add new encrypted arrays to the storage pool and then remove any non-encrypted arrays from the storage pool. The data on an array that you remove from a storage pool will automatically be moved to other arrays or managed disks in the pool.
When an array is removed from a storage pool, the member drives return to candidate status. A new array that is encrypted can be made with these drives.