<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2007/11/30/using-the-isbranch-method-to-determine-if-a-tree-item-is-a-branch-or-leaf/ -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        layout="vertical"
        verticalAlign="middle"
        backgroundColor="white">

<mx:Script>
        <![CDATA[
            import mx.events.ListEvent;

private function tree_itemClick(evt:ListEvent):void {
                var itemIsBranch:Boolean = tree.dataDescriptor.isBranch(tree.selectedItem);
                lbl.text = itemIsBranch.toString();
            }

private function tree_labelFunc(item:XML):String {
                var returnStr:String = item.@label;
                var itemIsBranch:Boolean = tree.dataDescriptor.isBranch(item);
                if (itemIsBranch) {
                    returnStr += " (BRANCH)";
                }
                return returnStr;
            }
        ]]>
    </mx:Script>

<mx:XML id="xmlDP">
        <node>
            <node label="1.a" />
            <node label="1.b" />
            <node label="1.c">
                <node label="1.c.i" />
                <node label="1.c.ii" />
                <node label="1.c.iii" />
                <node label="1.c.iv" />
                <node label="1.c.v" />
            </node>
            <node label="1.d" />
            <node label="1.e">
                <node label="1.e.i" />
                <node label="1.e.ii" />
                <node label="1.e.iii">
                    <node label="1.e.iii.A" />
                </node>
                <node label="1.e.iv" />
            </node>
            <node label="1.f" />
        </node>
    </mx:XML>

<mx:ApplicationControlBar dock="true">
        <mx:Form styleName="plain">
            <mx:FormItem label="isBranch():">
                <mx:Label id="lbl" fontWeight="bold" />
            </mx:FormItem>
        </mx:Form>
    </mx:ApplicationControlBar>

<mx:Tree id="tree"
            dataProvider="{xmlDP}"
            labelFunction="tree_labelFunc"
            showRoot="false"
            width="50%"
            rowCount="6"
            itemClick="tree_itemClick(event);" />

</mx:Application>