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.
Is the preview feature something you would consider adding in the future, either as part of the core plugin or as a paid one?