checked

Called without any parameters, the checked() method returns true if the RadioButton 's is checked otherwise returns false.
Calling it with a boolean true or false parameter will check or uncheck the corresponding RadioButton widget.

<input id="radiobutton" type="radio" ></div>

<script type="text/javascript">
    $(function () {
        $("#radiobutton").shieldRadioButton({
            label: "radiobutton 1"
        });
    });

    $("#radiobutton").swidget().checked(); // return false

    $("#radiobutton").swidget().checked(true) // check the widget

    $("#radiobutton").swidget().checked(); // return true
</script>