yyah this is easy.
what you do is run a search for your topic in google.com. then you click on the “More” option in the Google menu and choose “Discussions”. this will give you only results from inside forums.
another way is to search for your topic + forum and then check out the results.
what i like to do is find a very active forum to join that has audiences with similar interests to what i sell. so once you check the results, go to the main forum page and sort by “Number of responses” if you see several threads with 10,000+ views and 1000+ responses you’ve got a winner.