<div id="radiobuttonscontainer" runat="server">
    <div class="buttoncontainer">
        <asp:RadioButton ID="rb1" runat="server" GroupName="rbgroup"/>
    </div>
    <div class="buttoncontainer">
        <asp:RadioButton ID="rb2" runat="server" GroupName="rbgroup"/>
    </div>
</div>

var checkedButton = radiobuttonscontainer.Controls.OfType<RadioButton>()
                                  .FirstOrDefault(r => r.Checked);