Sorry for this question, but how will you be able to know if a blog/forum is a do follow or not? Thanks!
Check the source code of any page, if the <a href has nofollow in it then the site is using a system to block follow of the links. In robots.txt also it can be mentioned which sections of the site should not get crawled. You can also install firefox extension such as "searchstatus" which tells you which links are nofollowed.
Just download the SEO firefox and install in your firefox. When the link is highlighted that is no follow and if not that is Do follow.