Blueprint: how do I check if an optional dependency is satisfied?

In blueprint.xml, I declare an optional dependency this way:

<reference id="RepositoryListener"
    interface="ru.focusmedia.odp.server.datastore.api.RepositoryListener"
    availability="optional" />

<bean id="Repository"
    class="ru.focusmedia.odp.server.datastore.jpa.repository.RepositoryImpl">
    <jpa:context property="entityManager" unitname="ODP_Server" />
    <tx:transaction method="*" value="Required" />
    <property name="repositoryListener" ref="RepositoryListener" />
</bean>

and in RepositoryImpl, I have

public void setRepositoryListener(RepositoryListener repositoryListener) {
    logger.info("Repository listener set");
    this.repositoryListener = repositoryListener;
}

This method is called by Blueprint even when there is no RepositoryListener service available, as expected. The problem is, how can I check later whether there is a service?

    if (repositoryListener != null) {
        repositoryListener.notifyDelete(node);
    } else {
        logger.warn("No repository listener set!");
    }

doesn't work, since repositoryListener isn't null, but a Blueprint proxy.