It's easy to do this validation server side, but I am assuming you want to do it client side?
For more information on custom validators check out the following links: www.asp.net and
Here is an example I found at: Link to original
function ValidateModuleList(source, args)
var chkListModules= document.getElementById ('<%= chkModuleList.ClientID %>');
var chkListinputs = chkListModules.getElementsByTagName("input");
for (var i=0;i
Side Note: JQuery is just a little js file include you need to add to your page. Once you have it included you can use all the JQuery you like. Nothing to install and it will be full supported in the next version of Visual Studio I think.