def check_update ():
    """Return the following values:
       (False, errmsg) - online version could not be determined
       (True, None) - user has newest version
       (True, (version, url string)) - update available
       (True, (version, None)) - current version is newer than online version
    """
    version, value = get_online_version()
    if version is None:
        # value is an error message
        return False, value
    if version == CurrentVersion:
        # user has newest version
        return True, None
    if is_newer_version(version):
        # value is an URL linking to the update package
        return True, (version, value)
    # user is running a local or development version
    return True, (version, None)