<asp:RadioButtonList ID="RadioButtonList1" runat="server" RepeatDirection="Horizontal">
<asp:ListItem>MALE</asp:ListItem>
<asp:ListItem>FEMALE</asp:ListItem> </asp:RadioButtonList>
<asp:requiredfieldvalidator id="RequiredFieldValidator1" runat="server"
ControlToValidate="RadioButtonList1" ErrorMessage="Please Select"></asp:requiredfieldvalidator>
this is enough just add a requried filed validator give the radiobuttonlist list id as control to validate in the required field validator like the example above this would do validation both client side and server side it generates the javascript for client side