I've also tried to use cr-gpg to verify detached signatures. I'm getting the following message in a popup:
gpg: no valid OpenPGP data found.
gpg: the signature could not be verified.
Please remember that the signature file (.sig or .asc)
should be the first file given on the command line.
There is only one signature, signature.asc. It is valid. I do have the necessary public key in my local keyring. Can anyone else confirm that detached signature.asc files can be verified?