Full code after the jump.

View MXML

<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2008/11/10/detecting-when-the-number-of-radio-buttons-changes-in-a-radio-button-group-in-flex/ -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        layout="vertical"
        verticalAlign="middle"
        backgroundColor="white"
        initialize="init();">

    <mx:Script>
        <![CDATA[
            import mx.controls.Alert;

            private function init():void {
                radioGroup.addEventListener("numRadioButtonsChanged", radioGroup_numRadioButtonsChanged);
            }

            private function radioGroup_numRadioButtonsChanged(evt:Event):void {
                var rb:RadioButton = radioGroup.getRadioButtonAt(radioGroup.numRadioButtons-1);
                var alert:Alert = Alert.show(evt.toString(), "label:" + rb.label);
                alert.status = "numRadioButtons:" + radioGroup.numRadioButtons;
            }
        ]]>
    </mx:Script>

    <mx:RadioButtonGroup id="radioGroup" />

    <mx:HBox>
        <mx:RadioButton id="radioButton1" label="Red" group="{radioGroup}" />
        <mx:RadioButton id="radioButton2" label="Orange" group="{radioGroup}" />
        <mx:RadioButton id="radioButton3" label="Yellow" group="{radioGroup}" />
        <mx:RadioButton id="radioButton4" label="Green" group="{radioGroup}" />
        <mx:RadioButton id="radioButton5" label="Blue" group="{radioGroup}" />
    </mx:HBox>

</mx:Application>