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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.
LivePerson and LiveHelper both offer this functionality. If you aren't using the chat features though, just signing up for the visitor monitoring capabilities is a bit pricey.
There are a number of others, including one that represents you website as a neighborhood and your users as little graphical people. (I don't recall the name offhand though).
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.