<asp:RadioButtonList ID="rblradio" runat="server">

        <asp:ListItem Value="M" Text="Male"></asp:ListItem>

        <asp:ListItem Value="F" Text="Female"></asp:ListItem>

    </asp:RadioButtonList>

function ValidateAccessLevel() {

        var AccessLevelIndexValue = document.getElementById('<%=rblradio.ClientID%>');

         if (AccessLevelIndexValue.checked)
         {
            return false;

            }

        else
        {
            return true;
        }
    }

</script>

<asp:RequiredFieldValidator
    id="radioValidator
    runat="server"
    ControlToValidate="rblradio"
    Display="Dynamic">
    *
</asp:RequiredFieldValidator>