Ajax enabled session

I want to be able to determine if a user has javascript enabled for their browser. This way when they first visit my page I want set a session variable that will tell me if I can use ajax, or if I can't.

I was thinking of an ajax request, until I hear from the ajax request server side I assume the browser doesn't support javascript. Being that the request would be attempted on the first page load, I should know after a single page has been viewed.