I just ran across a watchdog column in the Dallas Morning News written by Jennifer LaFleur. She writes about open records and gives guidance on how to do your own investigative work.
This recent column goes a long way in explaining how to dig up info on non-profits. She spends some time on 990s and what can be gleaned from them.
She concludes with a list of ideas for what you can do to find more information about a non-profit.