How to ensure cookies are turned on.

Hello; I’m working on a website that uses sessions as an integral part of the site. If cookies are not turned on in the browser, my site will not work. I’ve been searching unsuccessfully to find the way to determine if the browser has cookies enabled. I want to warn the user that the site won’t work if they have cookies turned off. There must be some global variable that tells me. Could someone help me out? Thanks in advance, Gary