There's a hidden way to get detailed financial data on publicly traded U.S. banks days before the companies release their earnings. The information is accurate, free, and—most important—totally legal.

The source is the federal government, which publishes snapshots of banks' financial pictures, known as call reports, on a hard-to-find website. Banks, sometimes days later, distribute similar information in earnings releases that are followed closely by investors, analysts, and the media. On those occasions when call reports are published before earnings, savvy stock-pickers can glean useful information before other investors.

"This is a very unusual way to release earnings information," said Jeffrey Burks, a University of Notre Dame accounting professor who co-wrote a study on the practice. "You have a regulatory agency quietly putting it up on their website with no advance warning."