Wait, let me clarify you first the effects of forum postings at your site.
Forum postings gives you tons of backlinks. You can also gain traffics through forum.
Quality Forum Postings = Quality Backlinks + Visitors
Well, lets go back to your question, How to see the effects of Forums?
Simply check your backlinks to your backlink checker to see if your forum posts was credited by SE spiders. Another is, check your Statcounter account or Google Analytics to see if your forum posts contributed traffic to your site.
That's it! I hope it helps.
