I have been looking into this, but can't seem to find enough about it. I have heard a couple of things through word of mouth, and now want to know more!
The first thing I heard was at a clinic. The trainer mentioned that one horse had a bulge below her eyes, which is considered to be consistent with more stubborn horses, which this mare was.
The second thing I heard was that a horse who has small nostrils and a longer lip tend to be the more dominant, pushy type. That I know was true as well, because this time it was my horse :)
I am interested to hear more, as well as hear what people think about these 'old wive's tales'