I'm sorry but I didn't follow you. Do you want to be able moderate topics and posts before they become public? If so, please navigate to Dashboard > Forums > Settings > Forum Accesses admin page. Edit the "Registered" Usergroup, and disable/uncheck the "Can pass moderation" permission:
My last past was not the answer. I didn't follow you as I've already mentioned. There is no way to preview posts. My last post was related to Post Moderation/Review before publishing, it's not related to Preview so please ignore it. wpForo doesn't have preview feature.