We didn't have beta testing, but staff were the ones offering suggestions before we opened the forum to the public. The first couple months or so we got a lot of good critique from new members and various review services with ideas that we were able to implement.
I don't really test ideas for forums, I usually do my research, plan everything out and then start up and hope for the best. There isn't a lot of testing you can do for a forum, just improve as you go along with suggestions from other members.
I never do test before the implementation in ground level. If i has to develop a beautiful idea into a website. First of all I think of the how people will react and what make it more user friendly and easy. Then i publish my Idea on my AOI site, from where i get the suggestions and ideas and then i do the development till i'm not satisfied.