You can tell if the browser supports JS using
Page.Request.Browser.JavaScript. If you want to see if it's actually
*enabled*, it is a little harder; you could check if it's supported and
if so send a page containing a Javascript to redirect to a
Javascript-capable version of your page, which clearly would then only
be reached if script were actually enabled.
An example :
http://www.15seconds.com/issue/030303.htm
-- PH