BBC Radio 3 put it
The test involves turning to page 69 of each book, and judging its merits on the strength of the writing on that page.
Simple, and allegedly better than judging by the cover. The advice ran all around the British book press this year, apparently on the strength of John Sutherland's recommendation in How to Read a Novel: A User's Guide. Marshal Zeringue's Campaign for the American Reader has turned the test into a recurring feature, and you can find the latest installment here.