(Update 04-30-2020) The below CBS 60 minutes link are no longer valid. But I found a CNBC video from Harry on GE (another ponzi scheme, I agree).
(Original 03-12-2009, note the market low on 03-05-2009) Harry Markopolos blew the whistle on Madoff long time ago. But sadly SEC did not listen. Here is the CBS 60 minutes video.
Harry did a mathematical simultation to prove Madoff is a crook. For ordinary investors, I think of the signs I mentioned earlier (too good to be true; 10% annual compound return is very hard to achieve etc.)