Actually, take a look at SMF's implementation, it doesn't use AJAX/make additional requests, instead when you post a reply and someone else also has it will take you back to the preview post page and show a message along the lines of "Since you started typing your reply there have been new replies to this thread, you may wish to review your post."
Easiest way I can think of implementing it is to add a hidden field to the new reply form that contains the most recent PID, on submit check if there is a newer post in the thread than that PID, if there is show a message about it.