Interestingly enough, in 2009 Keith Braithwaite concocted a metric that can pretty reliably determine whether a system has been written with TDD or not.
See: http://peripateticaxiom.blogspot.com/2006/05/complexity-and-...
See: http://peripateticaxiom.blogspot.com/2006/05/complexity-and-...
However, the skeptic in me can't help but point out that this is not nearly rigorous enough to say his method can pretty reliably determine TDD vs non TDD code in the general case.