You can use appropriate firefox plugin to find if links are nofollow or dofollow. I forgot the name of it but if you search for it you can find it easily.
check the forum signature source and it may contians nofollow tags if the forum is nofollow otherwise nothing
Type in quick searchstatus into google. It's a firefox addon. When you've installed it right click it and select highlight no follow links.
I use SEOQuake plugging of firefox's, its handy and all information gather at a time, also save my time.
you may check manually by ctrl+U or right click of your mouse & click view source find rel="nofollow" attribute