Have you heard about AJAX? As soon as you enter a comment, you have to indicate that to the server which you do by submitting a request to the server. You then show the response that you get from the server on your page. You could either use Ajax or a normal submit.
SCJP 1.4, SCWCD 1.4 - Hints for you, Certified Scrum Master
Did a rm -R / to find out that I lost my entire Linux installation!