The quick answer is that you can't. The user only connects to your server at the point the page is requested. They're then working on a 'localised' version of the page with all the CF parsing already done.
You can check your logs to see when the last request for that page was, but that won't tell you when the next request is going to be.
Michael, I don't know if this is what you were looking for but try looking at [evilwalrus.com...] (site's not mine) in the php code archives. They have, I think, two php scripts (one flat-file, other requires SQL database) that track/output how many users are on a particular page at any given moment, though it prints this out on the page and anyone who's visiting that page can see.
dcrombie's response is correct, as far as real time goes, but some of the tracking software will give you recent visitors.
One that I like is PPHLOGGER. One of its features is to show "Currently Online Users". This feature provides visitor details over the last 30 minutes.
Since PPHLogger, measures time on each page and path through website, as well as exit pages, it is fairly accurate as to if a user is current. However, it would need to be making assumptions about when the exit occurs because people can simply close their browser window, or type another URL, rather then following an offsite link.