You have to use something other than Javascript to see if Javascript is turned on or not. If it's not, then the Javascript won't run to tell you it's not because the capabilities won't exist. You'll have to use some server side scripting to check this.
However, in this day and age, anyone who doesn't have Javascript turned on in their browser won't be able to do much at very many sites anyway. If you look at all the commercial websites that rely on Javascript for something significant in displaying things on the pages, you'll see most places DO use it. Anyone without Javascript turned on in their browser is going to either be used to or expecting seeing very little on the Internet, or so absolutely clueless they're not going to know the difference.