There is no real certain way to do it.. People can leave your site from that page, which will not register on your server. People can close the browser, which will not show. People may leave the computer and go for coffee.
that being said:
In the session log the time of arrival
Use javascript to load a page when they leave
In this page check the current time, and compare to the session time.
Tricky because: People may not have javascript enables.
Good luck!
