$('[data-val-required]').each(function () {
    // https://stackoverflow.com/a/13357474/8037
    var label = $('label[for="' + $(this).attr('id') + '"]');

    if (label.length == 0)
       label = $(this).closest('label');

    var text = label.text();

    if (text.length > 0)
        label.append('<span style="color:red"> *</span>');
});