Well at the moment only at the members page by counting. The statistics at the front page only show the number of active users. You could see it in the database, and there was a topic/mod that changed that line to X out of Y members have made Z posts....
You don't have to count them all by hand ofcourse if you use members.asp just multiply the number of pages with the number of members per page and correct that for the not completely full last page.