def _identify_loss_method(loss_method):
    """
    Identifies the symmetry for the given loss method.

    Parameters
    ----------
    loss_method : str
        The loss method to use. Should have the symmetry identifier as
        the prefix.

    Returns
    -------
    symmetric : bool
        True if `loss_method` had 's_' or 'symmetric_' as the prefix, else False.
    str
        The input `loss_method` value without the first section that indicated
        the symmetry.

    Raises
    ------
    ValueError
        Raised if the loss method does not have the correct form.

    """
    prefix, *split_method = loss_method.lower().split('_')
    if prefix not in ('a', 's', 'asymmetric', 'symmetric') or not split_method:
        raise ValueError('must specify loss function symmetry by appending "a_" or "s_"')
    if prefix in ('a', 'asymmetric'):
        symmetric = False
    else:
        symmetric = True
    return symmetric, '_'.join(split_method)