- Cheating - a lot of threats come from insiders who... probably made the tables!
 - Experience - when you've been in the business for a while you tend to notice trends.  users isn't there?  How about user, user_tbl or users_tbl?  Which ties into:
 - Educated Guesswork
 - And finally, more cheating - if the program is open source (wordpress, phpbb) or generally common (vbulletin) everything's available or documented.