1. Yes, vets can check for food allergies and also dust/mold/pollen allergies.
2. The dust/mold/pollen allergies were discovered a couple years prior to the food allergies. Rusty didn't have them until we moved to the Tennessee Valley, a/k/a Allergy Purgatory, according to my vet - lol And to top it all off, the vet said I am lucky enough to live on the worst end of the county for them.
I didn't have to have Rusty tested. Once the weathermen started talking about pollen count, Rusty's eyes and nose started watering and he was sneezing and coughing. The vet took one look at him, said "welcome to the Tennessee Valley" and put him on TriHist
Rusty was on Tri-Hist seasonally but I hated how it amped him up and even made his face look ugly. When I started buying herbs from Equine Science for my other horse's metabolic issues, I mentioned Rusty's airborne allergies.
He sent me a sample of his Herbal Allergy Blend for Rusty. I noticed a difference in 24 hours and within 72 hours his "sneezing/coughing/hacking" had completely disappeared and he was even a lot more calm.
Discovering the food allergies calmed him even more --- he is so close to a "normal" in-your-pocket TWH personality that it's scary
3. I discovered the food allergies thru my other horse that has metabolic issues. I had to take the metabolic horse completely off grain and any products with soy in them.
It was easier on me to feed all my horses the same as the metabolic horse, plus I knew it would be a healthy diet for everyone in that it was a low starch/non-soy diet.
That's how I found out about the food allergies and was kicking myself for being so stupid all those years with Rusty. That poor guy could have avoided a lot of whuppins if only I'd realized what was wrong.
I wish I had pictures of Rusty when he was being Mr. Evil Face because the softness in his eyes now is a completely different horse. He now enjoys being brushed (and I used a hard bristled plastic brush, plus the Furminator) and he loves to be vacuumed; neither of which appealed to him before food allergies were discovered.
I sure hope that's all it is with your Fella because it's an easy fix. He will probably still need some discipline from time-to-time.
Rusty has become a horse who can be 25' in front of me, going thru the stall gate into the main aisle and all I have to say is "Rusty whoa, step back; step back for a cookie" and Lord Love Him, he does exactly that. He could easily go right down aisle and out the other end of the barn, like the Arab does when he knows I can't reach him - lollol
EDITED TO ADD: Is the horse in your avatar the Fella we're talking about? He looks quite a bit like my Big Guy. This was Rusty's first ride in well over a year but he stood perfect at the trailer and went down the road like we'd been going every day. That is his "oh s**t, she really is going to make me work" look - lollol