How should a custom module self-update when it doesn't have a control panel?

Small issue I stumbled upon today.

When you have a custom module with an upd.module_name.php file, the file includes an update() function that is called to check if the module needs to be updated.

However, this function is only called when the control panel of the module is accessed. From the EE documentation:

This function is checked on any visit to the module’s control panel, and compares the current version number in the file to the recorded version in the database.

Modules can either have a control panel or not have a control panel depending on what you use for your ‘has_cp_backend’ flag (y/n).

If a module elects NOT to have a control panel (has_cp_backend = n), how can it tell EE that it is a new version and needs to update??