A Mirrored World -- How would we know?

I once attended a lecture where the speaker casually stated the following...

"If one was instantly transported to a mirror-image of our universe, in the absence of man-made influences, we would never see the difference,"

Immediately I figured out a method to determine which world I was in, but never had the chance to debate the issue with the author.  Has anyone else come across this 'challenge', and found a solution to it?