hook_update_N functions, Drupal stores last version of installed updates in his key-value storage, if update function executed without exceptions.
You can simply lookup installed version via command-line drush command:
drush ev "var_dump(drupal_get_installed_schema_version('my_module'))"
Sometimes you may need revert version to previous, for re-apply updates, or repeat updates, if they previously executed with problems.
You can set previous version of Drupal module schema version via this cli drush command: