def guess_identifier_format(identifier_str):
    """Guess identifier format.

    :param str identifier_str: Chemical identifier string.
    :return: 'inchi' or 'smiles' string.
    :rtype: :py:class:`str`
    """
    if identifier_str.startswith('InChI='):
        return 'inchi'
    else:
        return 'smiles'
