如何检索LI中的哪个单选按钮被选中

<ul class="ulGroup" runat="server" id="ulGroup">
    <li>
        <input type="radio" name="grouptype" id="rdGroup1" class="css-checkbox" value="Group 1" checked="checked" runat="server" />
        <label for="rdGroup1" class="css-label radGroup1">Group 1</label>
    </li>
    <li>
        <input type="radio" name="grouptype" id="rdGroup2" class="css-checkbox" value="Group 2" runat="server" />
        <label for="rdGroup2" class="css-label radGroup1">Group 2</label>
    </li>
    <li>
        <input type="radio" name="grouptype" id="rdGroup3" class="css-checkbox" value="Group 3" runat="server" />
        <label for="rdGroup3" class="css-label radGroup1">Group 3</label>
    </li>
    <li>
        <input type="radio" name="grouptype" id="rdGroup4" class="css-checkbox" value="Group 4" runat="server" />
        <label for="rdGroup4" class="css-label radGroup1">Group 4</label>
    </li>
</ul>
<ul class="ulGroup" runat="server" id="ulGroup"