Some signs are not blinking when you go near their eye (but sometimes that's hard to distinguish because they may blink from "feeling you" near their eye). I was told to throw a cotton ball near their face on the side you are worried about when they are standing calm and see if they react to that (But don't throw it on the eye, just near the face). A vet would TRULY be able to tell if the horse is blind or not. Is the horse's pupil dilated during the day? Or any forms of cloudiness? Those can be some signs.
John 14:6 - Jesus said unto him, "I am the way, the truth, and the life: no man cometh unto the Father, but by Me."