Sometimes a farrier or trimmer can have "papers" dripping off their finger tips and not be worth the powder to blow them to your state Capital and back
I know because I've had that unfortunate experience. The two brothers now trimming my horses have ten years under their belt, very little formal schooling, a ton of ethics/common sense.
They bent over backward to work WITH my vet, to trim my foundered horse after the "well-papered" SOB's cut him so short, he ended up with torn ligaments.
I would check his references and I would also ask him to part with the phone numbers of people who didn't like his work and ask them why they didn't like him.
Sometimes not liking the farrier or trimmer is un-founded - maybe he showed up late or there was a big personality conflict with him and the horse owner. Or maybe the horse ended up dead lame after the trim.
Unless you are very familiar with hoof trimming, there aren't a lot of questions you can ask him that he can't schmooze his way thru. He can pick up in a skinny minute, if you REALLY know your stuff or you're just blindly asking questions someone on a forum told you to ask
That is not the answer you were looking for but, I hope it helps