How do I tell if a disk image (.sparseimage) is encrypted?
from the command line you can use 'hdiutil isencrypted {image-name}'.

bash-3.2$ hdiutil isencrypted unencrypted.sparseimage
encrypted: NO
bash-3.2$ hdiutil isencrypted encrypted.sparseimage
encrypted: YES
blocksize: 512
uuid: FC3C26ED-3056-4F98-8248-DD2F16B39357
private-key-count: 0
passphrase-count: 1
max-key-count: 1
version: 2
bash-3.2$