Firstly, your horse should move better after the trim. If your horse is not going to move better after the trim, your trimmer should be able to explain why this is going to happen in language that you can understand, for how long it will last, and offer some options for pain relief (boots or other).
Your horse should in most instances walk away from the trim calmly, without hesitation, landing heel first and level headed. If your horse is not walking away better, the trimmer should have an action plan on how to improve your horse's movement.
The hairline at the coronet band should be even and smooth. It should not be bumpy, or wavy or dippy. This can change overnight, so you might want to make this assessment on the day after the trim.
Your trimmer should not be taking out sole unnecessarily. Sometimes bar material needs to be removed, but sole rarely needs to be taken out.
Your trimmer should be able to read the hoof and tell you about your horse's health generally.
Your horse should not be resisting the trim. If your horse is normally naughty, and you don't lift its feet at all between trims, then that's your fault. But if your horse is usually well behaved, but consistently badly behaved for the trimmer, and there are no other distractions, then your horse is trying to tell you something.Â
It's a bonus if your trimmer turns up when they are asked, and when they say they are going to, and is generally courteous to you and your horse, but at the bottom of this, your trimmer should genuinely care and be interested in your horse and how it's traveling.
Ultimately, regardless of what words come out of your trimmer's mouth, what jargon they use, or how good they tell you they are, your horse should have the final word on whether or not the trimmer is doing a good job. Ask your horse.
Are you getting a good trim?