There's page about horse eyesight, certainly interesting (but I find some info hard to believe -- really no one knows if horses can see colors?)
Anyway, it does not provide answer to one interesting question: "how well does this particular horse see"?
So, if you horse hits walls, you can probably figure out he has some problems, and there are other clues... like if you enter the stables and horse rotates his head 180 degrees -- that usually means that the eye originally facing you is blind or very weak.
Anyway, there's arabian mare called Hawkey, and she's nice, but I'm starting to suspect she's really short-sighted. Some objects, like stubs and logs near to the path, "scare" her: she'll decelerate suddenly, maybe change direction slightly to avoid the object, but then appears to look at the "scary" object and accelerate.
Today scary object was a bit of mud crossing the path. Then she tried to run away from a meadow -- unfortunately there was clearly visible fence in her escape path -- she appeared to only see it when it was like 2 meters away.
It is true that it makes her "interesting" to ride, but... (and it only happens when she goes first, so it is not that bad problem) I guess it would be nice to teach her not to do that...? (how?)
Is there some chance to verify how well her eyes actually work?
(I know there are quite big differences between horses; I have seen stallion not able to recognize "his" mare from 50 meters; and I have seen horse noticing fiber lying on the road... And many horses have problems finding apples on the ground... but is there some easy way to test horse's eyesight?)