By default the forum comes with some technical parameters that are not search engine friendly and therefore the forum driven pages will not be effectively indexed by Google and other major search engines.
On the other hand, you may discover that once your forum has finally been added to Google it has not been correctly indexed.
A very simple test is to browse over 4-5 forum pages from different posting dates, to select a few words phrase and run a Google search by enclosing your search phrase within quotes.
“this is the text extracted from my forum page that I want to search on Google”
This is a phpBB Search Engines Optimization sample searching phrase
To make it clearer about what this job entails, let us just mention a few of the technical parameters that are not search engines friendly: