Checking if entity exist in Google App Engine Datastore using JPA 1
It looks fine. You could omit the boolean flag if you return immediately when you know the answer:

public boolean isExist(final String genreName) throws PersistenceException {
    final EntityManager em = EMF.get().createEntityManager();
    try {
        final Genre genre = em.find(Genre.class, genreName);
        if (genre != null) {
            return true;
        }
        return false;
    } finally {
        em.close();
    }
}