<asp:RadioButtonList runat="server" ID="rdoDiscount" RepeatLayout="Flow" RepeatDirection="Horizontal">
        <asp:ListItem Value="1">折扣 </asp:ListItem>
        <asp:ListItem Value="2">价格 </asp:ListItem>
    </asp:RadioButtonList>
    <input type="button" onclick="haveChecked()?alert('已选择'):alert('未选择');" value="check" />

    <script type="text/javascript">
    function haveChecked()
    {
        var rbl=document.getElementById('<%=rdoDiscount.ClientID %>');
        var rbls=rbl.getElementsByTagName('input');
        for(var i=0;i<rbls.length;i++)
        {
            if(rbls[i].type=='radio')
                if(rbls[i].checked) return true;
        }
        return false;
    }
    
    </script>

------解决方案--------------------
改进一下

HTML code
<asp:RadioButtonList runat="server" ID="rdoDiscount" RepeatLayout="Flow" RepeatDirection="Horizontal">
        <asp:ListItem Value="1">折扣 </asp:ListItem>
        <asp:ListItem Value="2">价格 </asp:ListItem>
    </asp:RadioButtonList>
    <input type