I recall seeing a video about horse body language, many years ago...and the ears were mentioned. But the advice was general in nature...as in, "flat ears indicate that the horse is in a bad mood".
From what I see...the vast majority of the horses have their ears pointed forward, with the occasional flicker as a reaction to the oncoming sounds.
This ear-inspection advice can get a little contradictory though...
http://www.wikihow.com/Understand-Horse-Communication
