.panel-heading .accordion-toggle:after {
    /* symbol for "opening" panels */
    font-family: 'Glyphicons Halflings'; /* essential for enabling glyphicon */
    content: "\e114"; /* adjust as needed, taken from bootstrap.css */
    float: right; /* adjust as needed */
    color: grey; /* adjust as needed */
}

.panel-heading .accordion-toggle.collapsed:after {
    /* symbol for "collapsed" panels */
    content: "\e080"; /* adjust as needed, taken from bootstrap.css */
}