Is there a simple way to test vision so I can justify getting the vet out? My 19 YO has gotten a bit more spooky in the last several months, especially when he encounters "new" things (e.g., a fallen log in the woods that wasn't there before).
Simple obstacle course, put out a few things that he will need to walk around/over, poles the mounting block, bale of hay etc.. and on a loose lead take him to it, come at it from different directions and see what reactions you get. You have to be careful that you don't react to what you are walking toward, like slowing down your pace right before you get to the thing he needs to step over, take it at your normal rate of speed when you are leading him just be prepared for the reaction. Also try it at different times of day, dusk is usually a problem due to the low light. Good Luck, been there done that.
"Your life is an occasion...rise to it" Mr. Magorium
"They are dangerous on both ends and crafty in the middle" Sherlock Holmes- A Game of Shadows
The easiest possible test you can do is a menace reflex. Bring your hand toward the horse's eye quickly - they should blink. Google it for a better description of how to perform, but this is the absolute easiest way to check if an animal is visual.
The menace response is quick and easy, although if he has partial vision loss he may not necessarily show a difference. All he has to do is perceive the hand coming towards him.
You have to be careful that you do not accidentally hit the long hairs that stick out from the eye, or move your hand in such a way that you create an air current (don't come at him with a flat hand). But its basically a matter of quickly moving your fingers towards the animal's eye, so he thinks its going to hit it and blinks.
I would call the vet and have him or her do a good eye exam on your horse to rule out any problems, and have a Lyme Disease titer drawn at the same time. Lyme often goes up into the eyes and affects the vision. Spookiness is one of the signs of the disease.
Your justification in calling the vet would be your horse's health and your safety.