Web forum checker

I am wanting to create an app that will let a user check to see if their forum post has been replied to.

How should I go about this?

I was thinking to maybe check the content of the web page every 5 minutes. However, some pages have adds on them, so the content will change when a different add is displayed.

Should I maybe check the time of last write to a page? Will this work or will adds change the write time of the page or will the write time be only when the forum post is first created and will not be updated when a comment is replied to?

Kinda hard to say without some idea of what forum it is or, more importantly, the underlying program. All the major forum providers have APIs for this kind of thing and if you want a reliable application then you'll need to use them.

I'm having the same doubt as you. I want to create a desktop application on Vb.net 2010 to check if the user's post have been replied, if there are new messages, etc...
I want to know where can I start. The forum I want to use is the biggest of the country I live, so I think it will have some kind of api (like dunfiddlin) explains..
I know there have been a group of guys that have developed an extension for firefox that shows you the new messages. So, there should be a way to do the same task using a desktop application.
If someone knows more info, post it here.