mm-verify-option is a variable defined in `mm-decode.el'.
Its value is

Option of verifying signed parts.
`never', not verify; `always', always verify;
`known', only verify known protocols. Otherwise, ask user.

When set to `always' or `known', you should add
"multipart/signed" to `gnus-buttonized-mime-types' to see
result of the verification.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 22.1 of Emacs.