2 posts in the same discussion thread should consider as one link. 2 posts in two different discussion threads should be 2 links.
If you post two posts with website as anchor text. It will consider two links. For backlinks you can use backlinkwatch & opensite explorer.
There are many back link checker tool but i use SEO Quake. It shows all Google and yahoo back links. And very easy to use. So i like only SEO Quak.
Posting more than once in a forum can count as two backinks, as far as my experience goes. Use something like Market Samurai, or even something as simple as Yahoo Site Explorer. Both will give you a good idea of how many backlinks are pointing at your site, and from there, you can see how many are coming from the same forum, etc
Yahoo Site explorer, as in my experience this provides a very good tool for checking back links to your website. h__ps://siteexplorer.search.yahoo.com
link:http://www.(Your website).com/ in google will not show you all the links back to your website. Yahoo site explorer is a good place to start. I belive Majestic seo gives you a full report which you can download as well.