How would a submarine identify a hospital ship?

How would a submarine tell that a multi-role support hull was operating as a hospital ship?

The ship should broadcast and display its protected status, with the submarine using visual confirmation if necessary; compliance by both sides remains essential.

A protected hospital ship should broadcast its status and carry the appropriate designation and markings. A submarine may use its periscope when necessary to verify that identity before attacking. These signals reduce uncertainty when the same basic hull can serve in several roles. Legal markings provide protection only when the opponent follows the same rules.